Effective antibiotics include most macrolides, tetracyclines, ketolides, and quinolones. "Legionella" multiply within the cell, so any effective treatment must have excellent intracellular penetration. Current treatments of choice are the respiratory tract quinolones (levofloxacin, moxifloxacin, gemifloxacin) or newer macrolides (azithromycin, clarithromycin, roxithromycin). The antibiotics used most frequently have been levofloxacin, doxycycline, and azithromycin.

Macrolides (azithromycin) are used in all age groups, while tetracyclines (doxycycline) are prescribed for children above the age of 12 and quinolones (levofloxacin) above the age of 18. Rifampicin can be used in combination with a quinolone or macrolide. It is uncertain whether rifampicin is an effective antibiotic to take for treatment. The Infectious Diseases Society of America does not recommend the use of rifampicin with added regimens. Tetracyclines and erythromycin led to improved outcomes compared to other antibiotics in the original American Legion outbreak. These antibiotics are effective because they have excellent intracellular penetration in "Legionella"-infected cells. The recommended treatment is 5–10 days of levofloxacin or 3–5 days of azithromycin, but in people who are immunocompromised, have severe disease, or other pre-existing health conditions, longer antibiotic use may be necessary. During outbreaks, prophylactic antibiotics have been successfully used to prevent Legionnaires' disease in high-risk individuals who have possibly been exposed.

The mortality at the original American Legion convention in 1976 was high (29 deaths in 182 infected individuals) because the antibiotics used (including penicillins, cephalosporins, and aminoglycosides) had poor intracellular penetration. Mortality has plunged to less than 5% if therapy is started quickly. Delay in giving the appropriate antibiotic leads to higher mortality.

Although the risk of Legionnaires' disease being spread by large-scale water systems cannot be eliminated, it can be greatly reduced by writing and enforcing a highly detailed, systematic water safety plan appropriate for the specific type of facility involved (office building, hospital, hotel, spa, cruise ship, etc.) Some of the elements that such a plan may include are the following:

- Keeping water temperature either above or below the range in which the "Legionella" bacterium thrives.

- Preventing stagnation, for example by removing from a network of pipes any sections that have no outlet (dead ends). Where stagnation is unavoidable, for example when a wing of a hotel is closed for the off-season, systems must be thoroughly disinfected just prior to resuming normal operation.

- Preventing the buildup of biofilm, for example by not using (or by replacing) construction materials that encourage its development, and by reducing the quantity of nutrients for bacterial growth that enter the system.

- Periodic disinfection of the system, by high heat or a chemical biocide, and the use of chlorination where appropriate.

- System design (or renovation) that reduces the production of aerosols and reduces human exposure to them, for example by directing them well away from building air intakes.

An effective water safety plan will also cover such matters as training, record-keeping, communication among staff, contingency plans and management responsibilities. The format and content of the plan may be prescribed by public health laws or regulations. There is tentative evidence for the treatment of the water with copper-silver ionization or ultraviolet light.

The infection is treated with antibiotics. Tetracyclines and chloramphenicol are the drugs of choice for treating patients with psittacosis. Most persons respond to oral therapy doxycycline, tetracycline hydrochloride, or chloramphenicol palmitate. For initial treatment of severely ill patients, doxycycline hyclate may be administered intravenously. Remission of symptoms usually is evident within 48–72 hours. However, relapse can occur, and treatment must continue for at least 10–14 days after fever abates.

Neuraminidase inhibitors may be used to treat viral pneumonia caused by influenza viruses (influenza A and influenza B). No specific antiviral medications are recommended for other types of community acquired viral pneumonias including SARS coronavirus, adenovirus, hantavirus, and parainfluenza virus. Influenza A may be treated with rimantadine or amantadine, while influenza A or B may be treated with oseltamivir, zanamivir or peramivir. These are of most benefit if they are started within 48 hours of the onset of symptoms. Many strains of H5N1 influenza A, also known as avian influenza or "bird flu", have shown resistance to rimantadine and amantadine. The use of antibiotics in viral pneumonia is recommended by some experts, as it is impossible to rule out a complicating bacterial infection. The British Thoracic Society recommends that antibiotics be withheld in those with mild disease. The use of corticosteroids is controversial.

Antibiotics are the treatment of choice for bacterial pneumonia, with ventilation (oxygen supplement) as supportive therapy. The antibiotic choice depends on the nature of the pneumonia, the microorganisms most commonly causing pneumonia in the geographical region, and the immune status and underlying health of the individual. In the United Kingdom, amoxicillin is used as first-line therapy in the vast majority of patients acquiring pneumonia in the community, sometimes with added clarithromycin. In North America, where the "atypical" forms of community-acquired pneumonia are becoming more common, clarithromycin, azithromycin, or fluoroquinolones as single therapy have displaced the amoxicillin as first-line therapy.

Local patterns of antibiotic-resistance always need to be considered when initiating pharmacotherapy. In hospitalized individuals or those with immune deficiencies, local guidelines determine the selection of antibiotics.

In general, aspiration pneumonitis is treated conservatively with antibiotics indicated only for aspiration pneumonia. The choice of antibiotic will depend on several factors, including the suspected causative organism and whether pneumonia was acquired in the community or developed in a hospital setting. Common options include clindamycin, a combination of a beta-lactam antibiotic and metronidazole, or an aminoglycoside.

Corticosteroids are sometimes used in aspiration pneumonia, but there is limited evidence to support their effectiveness.

"Streptococcus pneumoniae" — amoxicillin (or erythromycin in patients allergic to penicillin); cefuroxime and erythromycin in severe cases.

"Staphylococcus aureus" — flucloxacillin (to counteract the organism's β-lactamase).

Usually initial therapy is empirical. If sufficient reason to suspect influenza, one might consider oseltamivir. In case of legionellosis, erythromycin or fluoroquinolone.

A third generation cephalosporin (ceftazidime) + carbapenems (imipenem) + beta lactam & beta lactamase inhibitors (piperacillin/tazobactam)

Treatment of CAP in children depends on the child's age and the severity of illness. Children under five are not usually treated for atypical bacteria. If hospitalization is not required, a seven-day course of amoxicillin is often prescribed, with co-trimaxazole an alternative when there is allergy to penicillins. Further studies are needed to confirm the efficacy of newer antibiotics. With the increase in drug-resistant Streptococcus pneumoniae, antibiotics such as cefpodoxime may become more popular. Hospitalized children receive intravenous ampicillin, ceftriaxone or cefotaxime, and a recent study found that a three-day course of antibiotics seems sufficient for most mild-to-moderate CAP in children.

In 2001 the American Thoracic Society, drawing on the work of the British and Canadian Thoracic Societies, established guidelines for the management of adult CAP dividing patients into four categories based on common organisms:

- Healthy outpatients without risk factors: This group (the largest) is composed of otherwise-healthy patients without risk factors for DRSP, enteric gram-negative bacteria, "pseudomonas" or other, less-common, causes of CAP. Primary microoganisms are viruses, atypical bacteria, penicillin-sensitive "streptococcus pneumoniae" and "haemophilus influenzae". Recommended drugs are macrolide antibiotics, such as azithromycin or clarithromycin, for seven to ten days.

- Outpatients with underlying illness or risk factors: Although this group does not require hospitalization, patients have underlying health problems (such as emphysema or heart failure) or are at risk for DRSP or enteric gram-negative bacteria. They are treated with a quinolone active against "streptococcus pneumoniae" (such as levofloxacin) or a β-lactam antibiotic (such as cefpodoxime, cefuroxime, amoxicillin or amoxicillin/clavulanic acid) and a macrolide antibiotic, such as azithromycin or clarithromycin, for seven to ten days.

- Hospitalized patients without risk for "pseudomonas": This group requires intravenous antibiotics, with a quinolone active against "streptococcus pneumoniae" (such as levofloxacin), a β-lactam antibiotic (such as cefotaxime, ceftriaxone, ampicillin/sulbactam or high-dose ampicillin plus a macrolide antibiotic (such as azithromycin or clarithromycin) for seven to ten days.

- Intensive-care patients at risk for "pseudomonas aeruginosa": These patients require antibiotics targeting this difficult-to-eradicate bacterium. One regimen is an intravenous antipseudomonal beta-lactam such as cefepime, imipenem, meropenem or piperacillin/tazobactam, plus an IV antipseudomonal fluoroquinolone such as levofloxacin. Another is an IV antipseudomonal beta-lactam such as cefepime, imipenem, meropenem or piperacillin/tazobactam, plus an aminoglycoside such as gentamicin or tobramycin, plus a macrolide (such as azithromycin) or a nonpseudomonal fluoroquinolone such as ciprofloxacin.

For mild-to-moderate CAP, shorter courses of antibiotics (3–7 days) seem to be sufficient.

Some patients with CAP will be at increased risk of death despite antimicrobial treatment. A key reason for this is the host's exaggerated inflammatory response. On one hand it is required to control the infection but on the other, it leads to bystander tissue damage. As a consequence of this recent research focuses on immunomodulatory therapy that can modulate the immune response to reduce injury to the lung and other affected organs such as the heart. Although the evidence for these agents has not resulted in their routine use, there potential benefits are highly promising.

Patients with HCAP are more likely than those with community-acquired pneumonia to receive inappropriate antibiotics that do not target the bacteria causing their disease.

In 2002, an expert panel made recommendations about the evaluation and treatment of probable nursing home-acquired pneumonia. They defined probably pneumonia, emphasized expedite antibiotic treatment (which is known to improve survival) and drafted criteria for the hospitalization of willing patients.

For initial treatment in the nursing home, a fluoroquinolone antibiotic suitable for respiratory infections (moxifloxacin, for example), or amoxicillin with clavulanic acid plus a macrolide has been suggested. In a hospital setting, injected (parenteral) fluoroquinolones or a second- or third-generation cephalosporin plus a macrolide could be used. Other factors that need to be taken into account are recent antibiotic therapy (because of possible resistance caused by recent exposure), known carrier state or risk factors for resistant organisms (for example, known carrier of MRSA or presence of bronchiectasis predisposing to Pseudomonas aeruginosa), or suspicion of possible Legionella pneumophila infection (legionnaires disease).

In 2005, the American Thoracic Society and Infectious Diseases Society of America have published guidelines suggesting antibiotics specifically for HCAP. The guidelines recommend combination therapy with an agent from each of the following groups to cover for both "Pseudomonas aeruginosa" and MRSA. This is based on studies using sputum samples and intensive care patients, in whom these bacteria were commonly found.

- cefepime, ceftazidime, imipenem, meropenem or piperacillin–tazobactam; plus

- ciprofloxacin, levofloxacin, amikacin, gentamicin, or tobramycin; plus

- linezolid or vancomycin

In one observational study, empirical antibiotic treatment that was not according to international treatment guidelines was an independent predictor of worse outcome among HCAP patients.

Guidelines from Canada suggest that HCAP can be treated like community-acquired pneumonia with antibiotics targeting Streptococcus pneumoniae, based on studies using blood cultures in different settings which have not found high rates of MRSA or Pseudomonas.

Besides prompt antibiotic treatment, supportive measure for organ failure (such as cardiac decompensation) are also important. Another consideration goes to hospital referral; although more severe pneumonia requires admission to an acute care facility, this also predisposes to hazards of hospitalization such as delirium, urinary incontinence, depression, falls, restraint use, functional decline, adverse drug effects and hospital infections. Therefore, mild pneumonia might be better dealt with inside the long term care facility. In patients with a limited life expectancy (for example, those with advanced dementia), end-of-life pneumonia also requires recognition and appropriate, palliative care.

Treatment of acute Q fever with antibiotics is very effective and should be given in consultation with an infectious diseases specialist. Commonly used antibiotics include doxycycline, tetracycline, chloramphenicol, ciprofloxacin, ofloxacin, and hydroxychloroquine. Chronic Q fever is more difficult to treat and can require up to four years of treatment with doxycycline and quinolones or doxycycline with hydroxychloroquine.

Q fever in pregnancy is especially difficult to treat because doxycycline and ciprofloxacin are contraindicated in pregnancy. The preferred treatment is five weeks of co-trimoxazole.

Most infections are mild and require no therapy or only symptomatic treatment. Because there is no virus-specific therapy, serious adenovirus illness can be managed only by treating symptoms and complications of the infection. Deaths are exceedingly rare but have been reported.

When infection attacks the body, "anti-infective" drugs can suppress the infection. Several broad types of anti-infective drugs exist, depending on the type of organism targeted; they include antibacterial (antibiotic; including antitubercular), antiviral, antifungal and antiparasitic (including antiprotozoal and antihelminthic) agents. Depending on the severity and the type of infection, the antibiotic may be given by mouth or by injection, or may be applied topically. Severe infections of the brain are usually treated with intravenous antibiotics. Sometimes, multiple antibiotics are used in case there is resistance to one antibiotic. Antibiotics only work for bacteria and do not affect viruses. Antibiotics work by slowing down the multiplication of bacteria or killing the bacteria. The most common classes of antibiotics used in medicine include penicillin, cephalosporins, aminoglycosides, macrolides, quinolones and tetracyclines.

Not all infections require treatment, and for many self-limiting infections the treatment may cause more side-effects than benefits. Antimicrobial stewardship is the concept that healthcare providers should treat an infection with an antimicrobial that specifically works well for the target pathogen for the shortest amount of time and to only treat when there is a known or highly suspected pathogen that will respond to the medication.

ILI occurs in some horses after intramuscular injection of vaccines. For these horses, light exercise speeds resolution of the ILI. Non-steroidal anti-inflammatory drugs (NSAIDs) may be given with the vaccine.

Safe and effective adenovirus vaccines were developed for adenovirus serotypes 4 and 7, but were available only for preventing ARD among US military recruits, and production stopped in 1996. Strict attention to good infection-control practices is effective for stopping transmission in hospitals of adenovirus-associated disease, such as epidemic keratoconjunctivitis. Maintaining adequate levels of chlorination is necessary for preventing swimming pool-associated outbreaks of adenovirus conjunctivitis.

Products containing multivalent cations, such as aluminium- or magnesium-containing antacids, and products containing calcium, iron or zinc invariably result in marked reduction of oral absorption of fluoroquinolones. Other drugs that interact with fluoroquinolones include sucralfate, probenecid, cimetidine, theophylline, warfarin, antiviral agents, phenytoin, cyclosporine, rifampin, pyrazinamide, and cycloserine.

Administration of quinolone antibiotics to a benzodiazepine dependent individual can precipitate acute benzodiazepine withdrawal symptoms due to quinolones displacing benzodiazepines from their binding site.

Fluoroquinolones have varying specificity for cytochrome P450, and so may have interactions with drugs cleared by those enzymes; the order from most P450-inhibitory to least, is enoxacin > ciprofloxacin > norfloxacin > ofloxacin, levofloxacin, trovafloxacin, gatifloxacin, moxifloxacin.

The mechanisms of the toxicity of fluoroquinolones have been attributed to their interactions with different receptor complexes, such as blockade of the GABAa receptor complex within the central nervous system, leading to excitotoxic type effects and oxidative stress.

Among the categories of bacteria most known to infect patients are the category MRSA (resistant strain of "S. aureus"), member of gram-positive bacteria and "Acinetobacter" ("A. baumannii"), which is gram-negative. While antibiotic drugs to treat diseases caused by gram-positive MRSA are available, few effective drugs are available for "Acinetobacter". "Acinetobacter" bacteria are evolving and becoming immune to existing antibiotics, so in many cases, polymyxin-type antibacterials need to be used. "In many respects it’s far worse than MRSA," said a specialist at Case Western Reserve University.

Another growing disease, especially prevalent in New York City hospitals, is the drug-resistant, gram-negative "Klebsiella pneumoniae". An estimated more than 20% of the "Klebsiella" infections in Brooklyn hospitals "are now resistant to virtually all modern antibiotics, and those supergerms are now spreading worldwide."

The bacteria, classified as gram-negative because of their reaction to the Gram stain test, can cause severe pneumonia and infections of the urinary tract, bloodstream, and other parts of the body. Their cell structures make them more difficult to attack with antibiotics than gram-positive organisms like MRSA. In some cases, antibiotic resistance is spreading to gram-negative bacteria that can infect people outside the hospital. "For gram-positives we need better drugs; for gram-negatives we need any drugs," said Dr. Brad Spellberg, an infectious-disease specialist at Harbor-UCLA Medical Center, and the author of "Rising Plague", a book about drug-resistant pathogens.

One-third of nosocomial infections are considered preventable. The CDC estimates 2 million people in the United States are infected annually by hospital-acquired infections, resulting in 20,000 deaths. The most common nosocomial infections are of the urinary tract, surgical site and various pneumonias.

Pontiac fever does not spread from person to person. It is acquired through aersolization of water droplets and/or potting soil containing "Legionella" bacteria.

Species of "Legionella" known to cause Pontiac fever include "Legionella pneumophila", "Legionella longbeachae", "Legionella feeleii", "Legionella micdadei", and "Legionella anisa".

General anaesthesia is recommended for people with sepsis who require surgical procedures to remove the infective source. Inhalational and intravenous anaesthetics are used. Requirements for anaesthetics may be reduced. Inhalational anaesthetics can reduce the level of proinflammatory cytokines, altering leukocyte adhesion and proliferation, inducing apoptosis (cell death) of the lymphocytes, possibly with a toxic effect on mitochondrial function. Although etomidate has a minimal effect on the cardiovascular system, it is often not recommended as a medication to help with intubation in this situation due to concerns it may lead to poor adrenal function and an increased risk of death. The small amount of evidence there is, however, has not found a change in the risk of death with etomidate.

It is recommended that the head of the bed be raised if possible to improve ventilation. Paralytic agents should be avoided unless ARDS is suspected.

Antibiotics do not help the many lower respiratory infections which are caused by parasites or viruses. While acute bronchitis often does not require antibiotic therapy, antibiotics can be given to patients with acute exacerbations of chronic bronchitis. The indications for treatment are increased dyspnoea, and an increase in the volume or purulence of the sputum. The treatment of bacterial pneumonia is selected by considering the age of the patient, the severity of the illness and the presence of underlying disease. Amoxicillin and doxycycline are suitable for many of the lower respiratory tract infections seen in general practice.

Early goal directed therapy (EGDT) is an approach to the management of severe sepsis during the initial 6 hours after diagnosis. It is a step-wise approach, with the physiologic goal of optimizing cardiac preload, afterload, and contractility. It includes giving early antibiotics. EGDT also involves monitoring of hemodynamic parameters and specific interventions to achieve key resuscitation targets which include maintaining a central venous pressure between 8–12 mmHg, a mean arterial pressure of between 65–90 mmHg, a central venous oxygen saturation (ScvO) greater than 70% and a urine output of greater than 0.5 ml/kg/hour. The goal is to optimize oxygen delivery to tissues and achieve a balance between systemic oxygen delivery and demand. An appropriate decrease in serum lactate may be equivalent to ScvO and easier to obtain.

In the original trial, early goal directed therapy was found to reduce mortality from 46.5% to 30.5% in those with sepsis, and the Surviving Sepsis Campaign has been recommending its use. However, three more recent large randomized control trials (ProCESS, ARISE, and ProMISe), did not demonstrate a 90-day mortality benefit of early goal directed therapy when compared to standard therapy in severe sepsis. It is likely that some parts of EGDT are more important than others. Following these trials the use of EGDT is still considered reasonable.

Recovery from an anaerobic infection depends on adequate and rapid management. The main principles of managing anaerobic infections are neutralizing the toxins produced by anaerobic bacteria, preventing the local proliferation of these organisms by altering the environment and preventing their dissemination and spread to healthy tissues.

Toxin can be neutralized by specific antitoxins, mainly in infections caused by Clostridia (tetanus and botulism). Controlling the environment can be attained by draining the pus, surgical debriding of necrotic tissue, improving blood circulation, alleviating any obstruction and by improving tissue oxygenation. Therapy with hyperbaric oxygen (HBO) may also be useful. The main goal of antimicrobials is in restricting the local and systemic spread of the microorganisms.

The available parenteral antimicrobials for most infections are metronidazole, clindamycin, chloramphenicol, cefoxitin, a penicillin (i.e. ticarcillin, ampicillin, piperacillin) and a beta-lactamase inhibitor (i.e. clavulanic acid, sulbactam, tazobactam), and a carbapenem (imipenem, meropenem, doripenem, ertapenem). An antimicrobial effective against Gram-negative enteric bacilli (i.e. aminoglycoside) or an anti-pseudomonal cephalosporin (i.e. cefepime ) are generally added to metronidazole, and occasionally cefoxitin when treating intra-abdominal infections to provide coverage for these organisms. Clindamycin should not be used as a single agent as empiric therapy for abdominal infections. Penicillin can be added to metronidazole in treating of intracranial, pulmonary and dental infections to provide coverage against microaerophilic streptococci, and Actinomyces.

Oral agents adequate for polymicrobial oral infections include the combinations of amoxicillin plus clavulanate, clindamycin and metronidazole plus a macrolide. Penicillin can be added to metronidazole in the treating dental and intracranial infections to cover "Actinomyces" spp., microaerophilic streptococci, and "Arachnia" spp. A macrolide can be added to metronidazole in treating upper respiratory infections to cover "S. aureus" and aerobic streptococci. Penicillin can be added to clindamycin to supplement its coverage against "Peptostreptococcus" spp. and other Gram-positive anaerobic organisms.

Doxycycline is added to most regimens in the treatment of pelvic infections to cover chlamydia and mycoplasma. Penicillin is effective for bacteremia caused by non-beta lactamase producing bacteria. However, other agents should be used for the therapy of bacteremia caused by beta-lactamase producing bacteria.

Because the length of therapy for anaerobic infections is generally longer than for infections due to aerobic and facultative anaerobic bacteria, oral therapy is often substituted for parenteral treatment. The agents available for oral therapy are limited and include amoxacillin plus clavulanate, clindamycin, chloramphenicol and metronidazole.

In 2010 the American Surgical Society and American Society of Infectious Diseases have updated their guidelines for the treatment of abdominal infections.

The recommendations suggest the following:

For mild-to-moderate community-acquired infections in adults, the agents recommended for empiric regimens are: ticarcillin- clavulanate, cefoxitin, ertapenem, moxifloxacin, or tigecycline as single-agent therapy or combinations of metronidazole with cefazolin, cefuroxime, ceftriaxone, cefotaxime, levofloxacin, or ciprofloxacin. Agents no longer recommended are: cefotetan and clindamycin ( Bacteroides fragilis group resistance) and ampicillin-sulbactam (E. coli resistance) and ainoglycosides (toxicity).

For high risk community-acquired infections in adults, the agents recommended for empiric regimens are: meropenem, imipenem-cilastatin, doripenem, piperacillin-tazobactam, ciprofloxacin or levofloxacin in combination with metronidazole, or ceftazidime or cefepime in combination with metronidazole. Quinolones should not be used unless hospital surveys indicate >90% susceptibility of "E. coli" to quinolones.

Aztreonam plus metronidazole is an alternative, but addition of an agent effective against gram-positive cocci is recommended. The routine use of an aminoglycoside or another second agent effective against gram-negative facultative and aerobic bacilli is not recommended in the absence of evidence that the infection is caused by resistant organisms that require such therapy.

Empiric use of agents effective against enterococci is recommended and agents effective against methicillin-resistant "S. aureus" (MRSA) or yeast is not recommended in the absence of evidence of infection due to such organisms.

Empiric antibiotic therapy for health care-associated intra-abdominal should be driven by local microbiologic results. Empiric coverage of likely pathogens may require multidrug regimens that include agents with expanded spectra of activity against gram-negative aerobic and facultative bacilli. These include meropenem, imipenem-cilastatin, doripenem, piperacillin-tazobactam, or ceftazidime or cefepime in combination with metronidazole. Aminoglycosides or colistin may be required.

Antimicrobial regimens for children include an aminoglycoside-based regimen, a carbapenem (imipenem, meropenem, or ertapenem), a beta-lactam/beta-lactamase-inhibitor combination (piperacillin-tazobactam or ticarcillin-clavulanate), or an advanced-generation cephalosporin (cefotaxime, ceftriaxone, ceftazidime, or cefepime) with metronidazole.

Clinical judgment, personal experience, safety and patient compliance should direct the physician in the choice of the appropriate antimicrobial agents. The length of therapy generally ranges between 2 and 4 weeks, but should be individualized depending on the response. In some instances treatment may be required for as long as 6–8 weeks, but can often be shortened with proper surgical drainage.