Immunodeficiency or immunosuppression can be caused by:

- Malnutrition

- Fatigue

- Recurrent infections

- Immunosuppressing agents for organ transplant recipients

- Advanced HIV infection

- Chemotherapy for cancer

- Genetic predisposition

- Skin damage

- Antibiotic treatment leading to disruption of the physiological microbiome, thus allowing some microorganisms to outcompete others and become pathogenic (e.g. disruption of intestinal flora may lead to "Clostridium difficile" infection

- Medical procedures

- Pregnancy

- Ageing

- Leukopenia (i.e. neutropenia and lymphocytopenia)

The lack of or the disruption of normal vaginal flora allows the proliferation of opportunistic microorganisms and will cause the opportunistic infection - bacterial vaginosis.

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

An individual may only develop signs of an infection after a period of subclinical infection, a duration that is called the incubation period. This is the case, for example, for subclinical sexually transmitted diseases such as AIDS and genital warts. Individuals with such subclinical infections, and those that never develop overt illness, creates a reserve of individuals that can transmit an infectious agent to infect other individuals. Because such cases of infections do not come to clinical attention, health statistics can often fail to measure the true prevalence of an infection in a population, and this prevents the accurate modeling of its infectious transmission.

Fever and sickness behavior and other signs of infection are often taken to be due to them. However, they are evolved physiological and behavioral responses of the host to clear itself of the infection. Instead of incurring the costs of deploying these evolved responses to infections, the body opts to tolerate an infection as an alternative to seeking to control or remove the infecting pathogen.

Subclinical infections are important since they allow infections to spread from a reserve of carriers. They also can cause clinical problems unrelated to the direct issue of infection. For example, in the case of urinary tract infections in women, this infection may cause preterm delivery if the person becomes pregnant without proper treatment.

Serratia infection refers to a disease caused by a species in the genus "Serratia".

The species involved is usually "Serratia marcescens".

It can cause nosocomial infections.

Sixty percent of mothers of preterm infants are infected with cytomegalovirus (CMV). Infection is asymptomatic in most instances but 9% to 12% of postnatally infected low birth weight, preterm infants have severe, sepsis-like infection. CMV infection duration can be long and result in pneumonitis in association with fibrosis. CMV infection in infants has an unexpected effect on the white blood cells of the immune system causing them to prematurely age. This leads to a reduced immune response similar to that found in the elderly.

Bats recovering from white-nose syndrome (WNS) may be the first natural occurrence of IRIS, in a report released by the USGS. WNS is typified by a cutaneous infection of the fungus "Pseudogymnoascus destructans" during hibernation, when the immune system is naturally suppressed to conserve energy through the winter. This study suggests that bats undergoing an intense inflammation at the site of infection after a return to euthermia is a form of IRIS.

Congential rubella is still a risk with higher risk among immigrant women from countries without adequate vaccination programs.

The current incidence in the United States is somewhere around 0.5% per year; overall, the incidence rate for developed world falls between 0.2–0.7%. In developing countries, the incidence of omphalitis varies from 2 to 7 for 100 live births. There does not appear to be any racial or ethnic predilection.

Like many bacterial infections, omphalitis is more common in those patients who have a weakened or deficient immune system or who are hospitalized and subject to invasive procedures. Therefore, infants who are premature, sick with other infections such as blood infection (sepsis) or pneumonia, or who have immune deficiencies are at greater risk. Infants with normal immune systems are at risk if they have had a prolonged birth, birth complicated by infection of the placenta (chorioamnionitis), or have had umbilical catheters.

Indwelling catheters have recently been identified with hospital acquired infections. Procedures using Intravascular Antimicrobial Lock Therapy can reduce infections that are unexposed to blood-borne antibiotics. Introducing antibiotics, including ethanol, into the catheter (without flushing it into the bloodstream) reduces the formation of biofilms.

Contact transmission is divided into two subgroups: direct-contact transmission and indirect-contact transmission.

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.

Most household disinfectants will inactivate FHV-1. The virus can survive up to 18 hours in a damp environment, but less in a dry environment and only shortly as an aerosol.

Most healthy people working with infants and children face no special risk from CMV infection. However, for women of child-bearing age who previously have not been infected with CMV, there is a potential risk to the developing unborn child (the risk is described above in the Pregnancy section). Contact with children who are in day care, where CMV infection is commonly transmitted among young children (particularly toddlers), may be a source of exposure to CMV. Since CMV is transmitted through contact with infected body fluids, including urine and saliva, child care providers (meaning day care workers, special education teachers, as well as mothers) should be educated about the risks of CMV infection and the precautions they can take. Day care workers appear to be at a greater risk than hospital and other health care providers, and this may be due in part to the increased emphasis on personal hygiene in the health care setting.

Recommendations for individuals providing care for infants and children:

- Employees should be educated concerning CMV, its transmission, and hygienic practices, such as handwashing, which minimize the risk of infection.

- Susceptible nonpregnant women working with infants and children should not routinely be transferred to other work situations.

- Pregnant women working with infants and children should be informed of the risk of acquiring CMV infection and the possible effects on the unborn child.

- Routine laboratory testing for CMV antibody in female workers is not specifically recommended due to its high occurrence, but can be performed to determine their immune status.

Gram-negative toe web infection is a relatively common infection. It is commonly found on people who are engaged in athletic activities while wearing closed-toe or tight fitting shoes. It grows in a moist environment. Gram-negative is mixed bacterial infection with the following organisms:

- Moraxella

- Alcaligenes

- Acinetobacter

- Pseudomonas

- Proteus

- Erwinia

This mixing of infection and organisms may also cause a mild secondary infection of tinea pedis.

Recommendations for pregnant women with regard to CMV infection:

- Throughout the pregnancy, practice good personal hygiene, especially handwashing with soap and water, after contact with diapers or oral secretions (particularly with a child who is in day care). Sharing of food, eating and drinking utensils, and contact with toddlers' saliva should be avoided.

- Women who develop a mononucleosis-like illness during pregnancy should be evaluated for CMV infection and counseled about the possible risks to the unborn child.

- Laboratory testing for antibody to CMV can be performed to determine if a woman has already had CMV infection.

- Recovery of CMV from the cervix or urine of women at or before the time of delivery does not warrant a cesarean section.

- The demonstrated benefits of breast-feeding outweigh the minimal risk of acquiring CMV from the breast-feeding mother.

- There is no need to either screen for CMV or exclude CMV-excreting children from schools or institutions because the virus is frequently found in many healthy children and adults.

Treatment with hyperimmune globulin in mothers with primary CMV infection has been shown to be effective in preventing congenital disease in several studies. One study did not show significant decrease in the risk of congenital cytomegalovirus infection.

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.

There is a vaccine for FHV-1 available (ATCvet code: , plus various combination vaccines), but although it limits or weakens the severity of the disease and may reduce viral shedding, it does not prevent infection with FVR. Studies have shown a duration of immunity of this vaccine to be at least three years. The use of serology to demonstrate circulating antibodies to FHV-1 has been shown to have a positive predictive value for indicating protection from this disease.

Infection in the newborn is accompanied by a strong immune response and is correlated with the need for prolonged mechanical ventilation.

Infection with "U. urealyticum" in pregnancy and birth can be complicated by chorioamnionitis, stillbirth, premature birth, and, in the perinatal period, pneumonia, bronchopulmonary dysplasia and meningitis. "U. urealyticum" has been found to be present in amniotic fluid in women who have had a premature birth with intact fetal membranes.

"U. urealyticum" has been noted as one of the infectious causes of sterile pyuria. It increases the morbidity as a cause of neonatal infections. It is associated with premature birth, preterm rupture of membranes, preterm labor, cesarean section, placental inflammation, congenital pneumonia, bacteremia, meningitis, fetal lung injury and death of infant. "Ureaplasma urealyticum" is associated with miscarriage.

Pyomyositis is most often caused by the bacterium "Staphylococcus aureus". The infection can affect any skeletal muscle, but most often infects the large muscle groups such as the quadriceps or gluteal muscles.

Pyomyositis is mainly a disease of children and was first described by Scriba in 1885. Most patients are aged 2 to 5 years, but infection may occur in any age group. Infection often follows minor trauma and is more common in the tropics, where it accounts for 4% of all hospital admissions. In temperate countries such as the US, pyomyositis was a rare condition (accounting for 1 in 3000 pediatric admissions), but has become more common since the appearance of the USA300 strain of MRSA.

IRIS is particularly problematic in cryptococcal meningitis as IRIS is fairly common and can be fatal.

IRIS has been described in immunocompetent hosts who have meningitis caused by "Cryptococcus gattii" and "Cryptococcus neoformans" var. "grubii", environmental fungi which often affect immunocompetent hosts. Several weeks or even months into appropriate treatment, there is a sudden onset deterioration with worsening meningitis symptoms and progression or development of new neurological symptoms.

Magnetic resonance imaging shows increase in the size of brain lesions, and CSF abnormalities (white cell count, protein, glucose) increase. CSF culture is typically sterile, and there is no increase in CSF cryptococcal antigen titer.

The increasing inflammation can cause brain injury or be fatal.

The general mechanism behind IRIS is increased inflammation as the recovering immune system recognizes the antigens of the fungus as immunosuppression is reversed. Cryptococcal IRIS has three phases:

1. before HAART, with a paucity of cerebrospinal fluid (CSF) inflammation and defects in antigen clearance;

2. during initial HAART immune recovery, with pro-inflammatory signaling by antigen-presenting cells without an effector response; and

3. at IRIS, a cytokine storm with a predominant type-1 helper T-cell interferon-gamma response.

Three clinical predictors of cryptococcal-related paradoxical IRIS risk include:

1. lack of initial CSF pleocytosis (i.e. low CSF white blood cell count);

2. elevated C-reactive protein;

3. failure to sterilize the CSF before immune recovery.

IRIS may be the cause of paradoxically worse outcomes for cryptococcal meningitis in immunocompetent compared with immunocompromised hosts, in whom "Cryptococcus neoformans" is the usual pathogen. Treatment with systemic corticosteroids during IRIS may be beneficial in preventing death or progressive neurological deterioration. Steroids given to persons with anti-fungal treatment failure / cryptococcal relapse (in whom CSF cultures are not sterile) can be a fatal iatrogenic error.

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.

Omphalitis is most commonly caused by bacteria. The culprits usually are "Staphylococcus aureus", "Streptococcus", and "Escherichia coli". The infection is typically caused by a combination of these organisms and is a mixed Gram-positive and Gram-negative infection. Anaerobic bacteria can also be involved.

One strategy for the prevention of infection transmission between cats and people is to better educate people on the behaviour that puts them at risk for becoming infected.

Those at the highest risk of contracting a disease from a cat are those with behaviors that include: being licked, sharing food, sharing kithchen utensils, kissing, and sleeping with a cat. The very young, the elderly and those who are immunocompromised increase their risk of becoming infected when sleeping with their cats (and dogs). The CDC recommends that cat owners not allow a cat to lick your face because it can result in disease transmission. If someone is licked on their face, mucous membranes or an open wound, the risk for infection is reduced if the area is immediately washed with soap and water. Maintaining the health of the animal by regular inspection for fleas and ticks, scheduling deworming medications along with veterinary exams will also reduce the risk of acquiring a feline zoonosis.

Recommendations for the prevention of ringworm transmission to people include:

- regularly vacuuming areas of the home that pets commonly visit helps to remove fur or flakes of skin

- washing the hands with soap and running water after playing with or petting your pet.

- wearing gloves and long sleeves when handling cats infected with.

- disinfect areas the pet has spent time in, including surfaces and bedding.

- the spores of this fungus can be killed with common disinfectants like chlorine bleach diluted 1:10 (1/4 cup in 1 gallon of water), benzalkonium chloride, or strong detergents.

- not handling cats with ringworm by those whose immune system is weak in any way (if you have HIV/AIDS, are undergoing cancer treatment, or are taking medications that suppress the immune system, for example).

- taking the cat to the veterinarian if ringworm infection is suspected.

Carrión's disease, or Oroya fever, or Peruvian wart is a rare infectious disease found only in Peru, Ecuador, and Colombia. It is endemic in some areas of Peru, is caused by infection with the bacterium "Bartonella bacilliformis", and transmitted by sandflies of genus "Lutzomyia".

Cat scratch disease occurs worldwide. Cats are the main reservoir of "Bartonella henselae", and the bacterium is transmitted to cats by the cat flea "Ctenocephalides felis". Infection in cats is very common with a prevalence estimated between 40-60%, younger cats being more commonly infective. Cats usually become immune to the infection, while dogs may be very symptomatic. Humans may also acquire it through flea or tick bites from infected dogs, cats, coyotes, and foxes.

Trench fever, produced by "Bartonella quintana" infection, is transmitted by the human body louse "Pediculus humanus corporis". Humans are the only known reservoir. Thorough washing of clothing may help to interrupt the transmission of infection.

A possible role for ticks in transmission of "Bartonella" species remains to be elucidated; in November 2011, "Bartonella rochalimae", "B. quintana", and "B. elizabethae" DNA was first reported in "Rhipicephalus sanguineus" and "Dermacentor nitens" ticks in Peru.

It had also been associated with a number of diseases in humans, including nonspecific urethritis, and infertility.