In Belgium the prevalence of nosocomial infections is about 6.2%. Annually about 125,500 patients become infected by a nosocomial infection, resulting in almost 3000 deaths. The extra costs for the health insurance are estimated to be approximately €400 million/year.

Estimates ranged from 6.7% in 1990 to 7.4% (patients may have several infections). At national level, prevalence among patients in health care facilities was 6.7% in 1996, 5.9% in 2001 and 5.0% in 2006. The rates for nosocomial infections were 7.6% in 1996, 6.4% in 2001 and 5.4% in 2006.

In 2006, the most common infection sites were urinary tract infections (30,3%), pneumopathy (14,7%), infections of surgery site (14,2%). Infections of the skin and mucous membrane (10,2%), other respiratory infections (6,8%) and bacterial infections / blood poisoning (6,4%). The rates among adult patients in intensive care were 13,5% in 2004, 14,6% in 2005, 14,1% in 2006 and 14.4% in 2007.

Nosocomial infections are estimated to make patients stay in the hospital four to five additional days. Around 2004-2005, about 9,000 people died each year with a nosocomial infection, of which about 4,200 would have survived without this infection.

Bacteria with resistance to antibiotics predate medical use of antibiotics by humans. However, widespread antibiotic use has made more bacteria resistant through the process of evolutionary pressure.

Reasons for the widespread use of antibiotics in human medicine include:

- increasing global availability over time since the 1950s

- uncontrolled sale in many low or middle income countries, where they can be obtained over the counter without a prescription, potentially resulting in antibiotics being used when not indicated. This may result in emergence of resistance in any remaining bacteria.

Other causes include:

- Antibiotic use in livestock feed at low doses for growth promotion is an accepted practice in many industrialized countries and is known to lead to increased levels of resistance.

- Releasing large quantities of antibiotics into the environment during pharmaceutical manufacturing through inadequate wastewater treatment increases the risk that antibiotic-resistant strains will develop and spread.

- It is uncertain whether antibacterials in soaps and other products contribute to antibiotic resistance, but antibacterial soaps are discouraged for other reasons.

The newborn`s exposure to the maternal vaginal bacterial flora which contains aerobic and anaerobic bacterial flora can lead to the development of anaerobic bacterial infection. These infections include cellulitis of the site of fetal monitoring (caused by "Bacterodes" spp.), bacteremia, aspiration pneumonia (caused by "Bacterodes" spp.), conjunctivitis (caused by clostridia,) omphalitis (caused by mixed flora), and infant botulism. Clostridial species may play a role in necrotizing enterocolitis. Management of these infection necessitates treating of the underlying condition(s) when present, and administration of proper antimicrobial therapy

The World Health Organization concluded that inappropriate use of antibiotics in animal husbandry is an underlying contributor to the emergence and spread of antibiotic-resistant germs, and that the use of antibiotics as growth promoters in animal feeds should be restricted. The World Organisation for Animal Health has added to the Terrestrial Animal Health Code a series of guidelines with recommendations to its members for the creation and harmonization of national antimicrobial resistance surveillance and monitoring programs, monitoring of the quantities of antibiotics used in animal husbandry, and recommendations to ensure the proper and prudent use of antibiotic substances. Another guideline is to implement methodologies that help to establish associated risk factors and assess the risk of antibiotic resistance.

Condition predisposing to anaerobic infections include: exposure of a sterile body location to a high inoculum of indigenous bacteria of mucous membrane flora origin, inadequate blood supply and tissue necrosis which lower the oxidation and reduction potential which support the growth of anaerobes. Conditions which can lower the blood supply and can predispose to anaerobic infection are: trauma, foreign body, malignancy, surgery, edema, shock, colitis and vascular disease. Other predisposing conditions include splenectomy, neutropenia, immunosuppression, hypogammaglobinemia, leukemia, collagen vascular disease and cytotoxic drugs and diabetes mellitus. A preexisting infection caused by aerobic or facultative organisms can alter the local tissue conditions and make them more favorable for the growth of anaerobes. Impairment in defense mechanisms due to anaerobic conditions can also favor anaerobic infection. These include production of leukotoxins (by "Fusobacterium" spp.), phagocytosis intracellular killing impairments (often caused by encapsulated anaerobes and by succinic acid ( produced by "Bacteroides" spp.), chemotaxis inhibition (by "Fusobacterium, Prevotella" and "Porphyromonas" spp.), and proteases degradation of serum proteins (by Bacteroides spp.) and production of leukotoxins (by "Fusobacterium" spp.).

The hallmarks of anaerobic infection include suppuration, establishment of an abscess, thrombophlebitis and gangrenous destruction of tissue with gas generation. Anaerobic bacteria are very commonly recovered in chronic infections, and are often found following the failure of therapy with antimicrobials that are ineffective against them, such as trimethoprim–sulfamethoxazole (co-trimoxazole), aminoglycosides, and the earlier quinolones.

Some infections are more likely to be caused by anaerobic bacteria, and they should be suspected in most instances. These infections include brain abscess, oral or dental infections, human or animal bites, aspiration pneumonia and lung abscesses, amnionitis, endometritis, septic abortions, tubo-ovarian abscess, peritonitis and abdominal abscesses following viscus perforation, abscesses in and around the oral and rectal areas, pus-forming necrotizing infections of soft tissue or muscle and postsurgical infections that emerge following procedures on the oral or gastrointestinal tract or female pelvic area. Some solid malignant tumors, ( colonic, uterine and bronchogenic, and head and neck necrotic tumors, are more likely to become secondarily infected with anaerobes. The lack of oxygen within the tumor that are proximal to the endogenous adjacent mucosal flora can predispose such infections.

A full spectrum of microorganisms is responsible for CAP in adults, and patients with certain risk factors are more susceptible to infections of certain groups of microorganisms. Identifying people at risk for infection by these organisms aids in appropriate treatment.

Many less-common organisms can cause CAP in adults, and are identified from specific risk factors or treatment failure for common causes.

Although children older than one month tend to be at risk for the same microorganisms as adults, children under five are much less likely to have pneumonia caused by "Mycoplasma pneumoniae", "Chlamydophila pneumoniae" or "Legionella pneumophila". In contrast, older children and teenagers are more likely to acquire "Mycoplasma pneumoniae" and "Chlamydophila pneumoniae" than adults.

While typical drug side effects reactions are mild to moderate; sometimes serious adverse effects occur.

As of 2016, the U.S. FDA recommended that "serious side effects associated with fluoroquinolone antibacterial drugs generally outweigh the benefits for patients with acute sinusitis, acute bronchitis, and uncomplicated urinary tract infections who have other treatment options. For patients with these conditions, fluoroquinolones should be reserved for those who do not have alternative treatment options."

Partly as a result of the efforts of Public Citizen, in 2008 the U.S. FDA ordered boxed warnings on all fluoroquinolones, advising consumers of an enhanced risk of tendon damage.

Prominent among these are side effects that became the subject of a black box warning by the U.S. FDA in 2016. The FDA wrote: "An FDA safety review has shown that fluoroquinolones when used systemically (i.e. tablets, capsules, and injectable) are associated with disabling and potentially permanent serious side effects that can occur together. These side effects can involve the tendons, muscles, joints, nerves, and central nervous system."

Quinolones are associated with a small risk of tendonitis and tendon rupture; a 2013 review found the incidence of tendon injury among those taking fluoroquinolones to be between 0.08 and 0.2%. The risk appears to be higher among people older than 60 and those also taking corticosteroids; there may also be higher risk among people who are male, have a pre-existing joint or tendon issue, have kidney disease, and are highly active. Some experts have advised avoidance of fluoroquinolones in athletes. If tendonitis occurs, it generally appears within one month, and the most common tendon that is injured appears to be the Achilles tendon. The cause is not well understood.

Nervous system effects include insomnia, restlessness, and rarely, seizure, convulsions, and psychosis. Other rare and serious adverse events have been observed with varying degrees of evidence for causation.

More generally, fluoroquinolones are tolerated, with typical drug side effects being mild to moderate. Common side effects include gastrointestinal effects such as nausea, vomiting, and diarrhea, as well as headache and insomnia. Postmarketing surveillance has revealed a variety of relatively rare but serious adverse effects that are associated with all members of the fluoroquinolone antibacterial class. Among these, tendon problems and exacerbation of the symptoms of the neurological disorder myasthenia gravis are the subject of "black box" warnings in the United States.

The overall rate of adverse events in patients treated with fluoroquinolones is roughly similar to that seen in patients treated with other antibiotic classes. A U.S. Centers for Disease Control and Prevention study found patients treated with fluoroquinolones experienced adverse events severe enough to lead to an emergency department visit more frequently than those treated with cephalosporins or macrolides, but less frequently than those treated with penicillins, clindamycin, sulfonamides, or vancomycin.

Fluoroquinolones prolong the heart's QT interval by blocking voltage-gated potassium channels. Prolongation of the QT interval can lead to torsades de pointes, a life-threatening arrhythmia, but in practice this appears relatively uncommon in part because the most widely prescribed fluoroquinolones (ciprofloxacin and levofloxacin) only minimally prolong the QT interval.

"Clostridium difficile" colitis may occur in connection with the use of any antibacterial drug, especially those with a broad spectrum of activity such as clindamycin, cephalosporins, and fluoroquinolones. Fluoroquinoline treatment is associated with risk that is similar to or less than that associated with broad spectrum cephalosporins. Fluoroquinoline administration may be associated with the acquisition and outgrowth of a particularly virulent "Clostridium" strain.

Events that may occur in acute overdose are rare, and include renal failure and seizure. Susceptible groups of patients, such as children and the elderly, are at greater risk of adverse reactions during therapeutic use.

In 2017 the FDA included the following important warning:

"The U.S. Food and Drug Administration (FDA) has required the drug labels and Medication Guides for all fluoroquinolone antibacterial drugs be updated to better describe the serious side effect of peripheral neuropathy. This serious nerve damage potentially caused by fluoroquinolones (see Table for a list) may occur soon after these drugs are taken and may be permanent.

The risk of peripheral neuropathy occurs only with fluoroquinolones that are taken by mouth or by injection. Approved fluoroquinolone drugs include levofloxacin (Levaquin), ciprofloxacin (Cipro), moxifloxacin (Avelox), norfloxacin (Noroxin), ofloxacin (Floxin), and gemifloxacin (Factive). The topical formulations of fluoroquinolones, applied to the ears or eyes, are not known to be associated with this risk.

If a patient develops symptoms of peripheral neuropathy, the fluoroquinolone should be stopped, and the patient should be switched to another, non-fluoroquinolone antibacterial drug, unless the benefit of continued treatment with a fluoroquinolone outweighs the risk. Peripheral neuropathy is a nerve disorder occurring in the arms or legs. Symptoms include pain, burning, tingling, numbness, weakness, or a change in sensation to light touch, pain or temperature, or the sense of body position. It can occur at any time during treatment with fluoroquinolones and can last for months to years after the drug is stopped or be permanent. Patients using fluoroquinolones who develop any symptoms of peripheral neuropathy should tell their health care professionals right away.

FDA will continue to evaluate the safety of drugs in the fluoroquinolone class and will communicate with the public again if additional information becomes available. "

In most countries, fluoroquinolones are approved for use in children only under narrowly-defined circumstances, owing in part to the observation of high rates of musculoskeletal adverse events in fluoroquinolone treated juvenile animals. In the UK, the prescribing indications for fluoroquinolones for children are severely restricted. Only inhalant anthrax and pseudomonal infections in cystic fibrosis infections are licensed indications in the UK due to ongoing safety concerns. In a study comparing the safety and efficacy of levofloxacin to that of azithromycin or ceftriaxone in 712 children with community-acquired pneumonia, serious adverse events were experienced by 6% of those treated with levofloxacin and 4% of those treated with comparator antibiotics. Most of these were considered by the treating physician to be unrelated or doubtfully related to the study drug. Two deaths were observed in the levofloxacin group, neither of which was thought to be treatment-related. Spontaneous reports to the U.S. FDA Adverse Effects Reporting System at the time of the 20 September 2011 U.S. FDA Pediatric Drugs Advisory Committee included musculoskeletal events (39, including 5 cases of tendon rupture) and central nervous system events (19, including 5 cases of seizures) as the most common spontaneous reports between April 2005 and March 2008. An estimated 130,000 pediatric prescriptions for levofloxacin were filled on behalf of 112,000 pediatric patients during that period.

Meta-analyses conclude that fluoroquinolones pose little or no additional risk to children compared to other antibiotic classes.

Fluoroquinolines use in children may be appropriate when the infection is caused by multidrug-resistant bacteria, or when alternative treatment options require parenteral administration and oral therapy is preferred.

Travelers often get diarrhea from eating and drinking foods and beverages that have no adverse effects on local residents. This is due to immunity that develops with constant, repeated exposure to pathogenic organisms. The extent and duration of exposure necessary to acquire immunity has not been determined; it may vary with each individual organism. A study among expatriates in Nepal suggests that immunity may take up to seven years to develop—presumably in adults who avoid deliberate pathogen exposure.

Conversely, immunity acquired by American students while living in Mexico disappeared, in one study, as quickly as eight weeks after cessation of exposure.

The primary source of infection is ingestion of fecally contaminated food or water. Attack rates are similar for men and women.

The most important determinant of risk is the traveler's destination. High-risk destinations include developing countries in Latin America, Africa, the Middle East, and Asia. Among backpackers, additional risk factors include drinking untreated surface water and failure to maintain personal hygiene practices and clean cookware. Campsites often have very primitive (if any) sanitation facilities, making them potentially as dangerous as any developing country.

Although traveler's diarrhea usually resolves within three to five days (mean duration: 3.6 days), in about 20% of cases, the illness is severe enough to require bedrest, and in 10%, the illness duration exceeds one week. For those prone to serious infections, such as bacillary dysentery, amoebic dysentery, and cholera, TD can occasionally be life-threatening. Others at higher-than-average risk include young adults, immunosuppressed persons, persons with inflammatory bowel disease or diabetes, and those taking H2 blockers or antacids.

Opportunistic infections caused by Feline Leukemia Virus and Feline immunodeficiency virus retroviral infections can be treated with Lymphocyte T-Cell Immune Modulator.

Since opportunistic infections can cause severe disease, much emphasis is placed on measures to prevent infection. Such a strategy usually includes restoration of the immune system as soon as possible, avoiding exposures to infectious agents, and using antimicrobial medications ("prophylactic medications") directed against specific infections.

Humans contract "Blastocystis" infection by drinking water or eating food contaminated with feces from an infected human or animal. "Blastocystis" infection can be spread from animals to humans, from humans to other humans, from humans to animals, and from animals to animals. Risk factors for infection have been reported as following:

- International travel: Travel to less developed countries has been cited in development of symptomatic Blastocystis infection. A 1986 study in the United States found that all individuals symptomatically infected with "Blastocystis" reported recent travel history to less developed countries. In the same study, all hospital employees working in New York who were screened for "Blastocystis" were found to have asymptomatic infections.

- Military service: Several studies have identified high rates of infection in military personnel. An early account described infection of British troops in Egypt in 1916 who recovered following treatment with emetine. A 1990 study published in "Military Medicine" from Lackland AFB in Texas concluded symptomatic infection was more common in foreign nationals, children, and immunocompromised individuals. A 2002 study published in "Military Medicine" of army personnel in Thailand identified a 44% infection rate. Infection rates were highest in privates who had served the longest at the army base. A follow-up study found a significant correlation between infection and symptoms, and identified the most likely cause as contaminated water. A 2007 newspaper article suggested the infection rate of US military personnel returning from the Gulf War was 50%, quoting the head of Oregon State University's Biomedicine department.

- Consumption of Untreated Water (well water): Many studies have linked "Blastocystis" infection with contaminated drinking water. A 1993 study of children infected symptomatically with "Blastocystis" in Pittsburgh indicated that 75% of them had a history of drinking well water or travel in less developed countries. Two studies in Thailand linked "Blastocystis" infection in military personnel and families to drinking of unboiled and untreated water. A book published in 2006 noted that in an Oregon community, infections are more common in winter months during heavy rains. A research study published in 1980 reported bacterial contamination of well water in the same community during heavy rainfall. A 2007 study from China specifically linked infection with "Blastocystis sp. subtype 3" with drinking untreated water. Recreational contact with untreated water, for example though boating, has also been identified as a risk factor. Studies have shown that "Blastocystis" survives sewage treatment plants in both the United Kingdom and Malaysia. "Blastocystis" cysts have been shown to be resistant to chlorination as a treatment method and are among the most resistant cysts to ozone treatment.

- Contaminated Food: Contamination of leafy vegetables has been implicated as a potential source for transmission of "Blastocystis" infection, as well as other gastrointestinal protozoa. A Chinese study identified infection with "Blastocystis sp. subtype 1" as specifically associated with eating foods grown in untreated water.

- Daycare facilities: A Canadian study identified an outbreak of "Blastocystis" associated with daycare attendance. Prior studies have identified outbreaks of similar protozoal infections in daycares.

- Geography: Infection rates vary geographically, and variants which produce symptoms may be less common in industrialized countries. For example, a low incidence of "Blastocystis" infection has been reported in Japan. A study of individuals infected with "Blastocystis" in Japan found that many (43%, 23/54) carried "Blastocystis sp. subtype 2", which was found to produce no symptoms in 93% (21/23) of patients studied, in contrast to other variants which were less common but produced symptoms in 50% of Japanese individuals. Studies in urban areas of industrialized countries have found "Blastocystis" infection associated with a low incidence of symptoms. In contrast, studies in developing countries generally show "Blastocystis" to be associated with symptoms. In the United States, a higher incidence of "Blastocystis" infection has been reported in California and West Coast states.

- Prevalence over Time: A 1989 study of the prevalence of "Blastocystis" in the United States found an infection rate of 2.6% in samples submitted from all 48 states. The study was part of the CDC's MMWR Report. A more recent study, in 2006, found an infection rate of 23% in samples submitted from all 48 states. However, the more recent study was performed by a private laboratory located in the Western US, and emphasized samples from Western states, which have previously been reported to have a higher infection rate.

Research studies have suggested the following items are not risk factors for contracting "Blastocystis" infection:

- Consumption of municipal water near water plant (not a risk factor): One study showed that municipal water was free of "Blastocystis", even when drawn from a polluted source. However, samples taken far away from the treatment plant showed cysts. The researchers suggested that aging pipes may permit intrusion of contaminated water into the distribution system.

- Human-to-Human transmission among adults (not a risk factor): Some research suggests that direct human-to-human transmission is less common even in households and between married partners. One study showed different members of the same household carried different subtypes of Blastocystis.

Early onset sepsis can occur in the first week of life. It usually is apparent on the first day after birth. This type of infection is usually acquired before the birth of the infant. Premature rupture of membranes and other obstetrical complications can add to the risk of early-onset sepsis. If the amniotic membrane has been ruptured greater than 18 hours before delivery the infant may be at more risk for this complication. Prematurity, low birth weight, chorioamnionitis, maternal urinary tract infection and/or maternal fever are complications that increase the risk for early-onset sepsis. Early onset sepsis is indicated by serious respiratory symptoms. The infant usually suffers from pneumonia, hypothermia, or shock. The mortality rate is 30 to 50%.

Infections that occur after the first week of life but before the age of 30 days are considered late onset infections. Obstetrical and maternal complications are not typically the cause of these late onset infections; they are usually acquired by the infant in the hospital neonatal intensive care unit. The widespread use of broad-spectrum antibiotics in the nursery intensive care unit can cause a higher prevalence of invasive antibiotic resistant bacteria. Meconium aspiration syndrome has a mortality rate just over 4%. This accounts for 2% for all neonatal deaths.

A study conducted on 452 patients revealed that the genotype responsible for higher IL-10 expression makes HIV infected people more susceptible to tuberculosis infection. Another study on HIV-TB co-infected patients also concluded that higher level of IL-10 and IL-22 makes TB patient more susceptible to Immune reconstitution inflammatory syndrome (IRIS). It is also seen that HIV co-infection with tuberculosis also reduces concentration of immunopathogenic matrix metalloproteinase (MMPs) leading to reduced inflammatory immunopathology.

Experimental infection in immunocompetent and immunocompromised mice has produced intestinal inflammation, altered bowel habits, lethargy and death. Chronic diarrhea has been reported in non-human higher primates.

To date, the precise causative factor has not been verified, and the disease has been attributed by various sources to viruses, parasites, bacteria, use of antibiotics and sulfonamides, and heavy metal poisoning. Other possible causes include peracute salmonellosis, clostridial enterocolitis, and endotoxemia. "Clostridium difficile" toxins isolated in the horse have a genotype like the current human "epidemic strain", which is associated with human "C. difficile"-associated disease of greater than historical severity. "C. difficile" can cause pseudomembranous colitis in humans, and in hospitalized patients who develop it, fulminant "C. difficile" colitis is a significant and increasing cause of death.

Horses under stress appear to be more susceptible to developing colitis X. Disease onset is often closely associated with surgery or transport. Excess protein and lack of cellulose content in the diet (a diet heavy on grain and lacking adequate hay or similar roughage) is thought to be the trigger for the multiplication of clostridial organisms. A similar condition may be seen after administration of tetracycline or lincomycin to horses. These factors may be one reason the condition often develops in race horses, having been responsible for the deaths of the Thoroughbred filly Landaluce,

the Quarter Horse stallion Lightning Bar,

and is one theory for the sudden death of Kentucky Derby winner Swale.

The link to stress suggests the condition may be brought on by changes in the microflora of the cecum and colon that lower the number of anaerobic bacteria, increase the number of Gram-negative enteric bacteria, and decrease anaerobic fermentation of soluble carbohydrates, resulting in damage to the cecal and colonic mucosa and allowing increased absorption of endotoxins from the lumen of the gut.

The causative agent may be "Clostridium perfringens", type A, but the bacteria are recoverable only in the preliminary stages of the disease.

The suspect toxin could also be a form of "Clostridium difficile". In a 2009 study at the University of Arizona, "C. difficile" toxins A and B were detected, large numbers of "C. difficile" were isolated, and genetic characterization revealed them to be North American pulsed-field gel electrophoresis type 1, polymerase chain reaction ribotype 027, and toxinotype III. Genes for the binary toxin were present, and toxin negative-regulator tcdC contained an 18-bp deletion. The individual animal studied in this case was diagnosed as having peracute typhlocolitis, with lesions and history typical of those attributed to colitis X.

Use of antibiotics may also be associated with some forms of colitis-X. In humans, "C. difficile" is the most serious cause of antibiotic-associated diarrhea, often a result of eradication of the normal gut flora by antibiotics. In one equine study, colitis was induced after pretreatment with clindamycin and lincomycin, followed by intestinal content from horses which had died from naturally occurring idiopathic colitis. (A classic adverse effect of clindamycin in humans is "C. difficile"-associated diarrhea.) In the experiment, the treated horses died. After necropsy, "Clostridium cadaveris" was present, and is proposed as another possible causative agent in some cases of fatal colitis.

While the number of penicillin-resistant bacteria is increasing, penicillin can still be used to treat a wide range of infections caused by certain susceptible bacteria, including Streptococci, Staphylococci, Clostridium, and Listeria genera. The following list illustrates minimum inhibitory concentration susceptibility data for a few medically significant bacteria:

- "Listeria monocytogenes": from less than or equal to 0.06 μg/ml to 0.25 μg/ml

- "Neisseria meningitidis": from less than or equal to 0.03 μg/ml to 0.5 μg/ml

- "Staphylococcus aureus": from less than or equal to 0.015 μg/ml to more than 32 μg/ml

A superinfection is a second infection superimposed on an earlier one, especially by a different microbial agent of exogenous or endogenous origin, that is resistant to the treatment being used against the first infection. Examples of this in bacteriology are the overgrowth of endogenous "Clostridium difficile" which occurs following treatment with a broad-spectrum antibiotic, and pneumonia or septicemia from "Pseudomonas aeruginosa" in some immuno-compromised patients.

In virology, the definition is slightly different. Superinfection is the process by which a cell that has previously been infected by one virus gets co-infected with a different strain of the virus, or another virus, at a later point in time. Viral superinfections may be resistant to the antiviral drug or drugs that were being used to treat the original infection. Viral superinfections may also be less susceptible to the host's immune response.

Antibiotic-associated diarrhea (AAD) results from an imbalance in the colonic microbiota caused by antibiotic therapy. Microbiota alteration changes carbohydrate metabolism with decreased short-chain fatty acid absorption and an osmotic diarrhea as a result. Another consequence of antibiotic therapy leading to diarrhea is overgrowth of potentially pathogenic organisms such as "Clostridium difficile". It is defined as frequent loose and watery stools with no other complications.

Meta-analyses have concluded that probiotics may protect against antibiotic-associated diarrhea in both children and adults. Evidence is insufficient, however, regarding an effect on rates of "Clostridium difficile" colitis.

However, citing conflicting data in the studies, other sources claim that the use of probiotics has failed thus far to meet the standard of medical care required for evidence-based medicine. Demonstration of the efficacy of probiotics is needed by randomized, double blind, placebo-controlled trials.

Efficacy of probiotic AAD prevention is dependent on the probiotic strain(s) used and on the dosage. Up to a 50% reduction of AAD occurrence has been found. No side-effects have been reported in any of these studies. Caution should, however, be exercised when administering probiotic supplements to immunocompromised individuals or patients who have a compromised intestinal barrier because of the risk of an infection caused by the probiotic supplements.

"Clostridium difficile", also known more commonly as "C. diff", is known to account for 10 to 20 percent of antibiotic-associated diarrhea cases. The reasoning for this, is that the antibiotics administered for the treatment of certain diseases processes such as inflammatory colitis also inadvertently kills a large portion of the gut flora, the normal flora that is usually present within the bowel. With this lower amount of "healthy" bacteria present, the overgrowth of "C. diff" is then responsible "for elaborating the enterotoxin".

Colitis X, equine colitis X or peracute toxemic colitis is a catchall term for various fatal forms of acute or peracute colitis found in horses, but particularly a fulminant colitis where clinical signs include sudden onset of severe diarrhea, abdominal pain, shock, and dehydration. Death is common, with 90% to 100% mortality, usually in less than 24 hours. The causative factor may be "Clostridium difficile", but it also may be caused by other intestinal pathogens. Horses under stress appear to be more susceptible to developing colitis X, and like the condition pseudomembranous colitis in humans, there also is an association with prior antibiotic use. Immediate and aggressive treatment can sometimes save the horse, but even in such cases, 75% mortality is considered a best-case scenario.

"Candida albicans", a yeast, is associated with endocarditis in IV drug users and immunocompromised patients. Other fungi demonstrated to cause endocarditis are "Histoplasma capsulatum" and Aspergillus. Endocarditis with "Tricosporon asahii" has also been reported in a case report.