Pneumonia is due to infections caused primarily by bacteria or viruses and less commonly by fungi and parasites. Although there are more than 100 strains of infectious agents identified, only a few are responsible for the majority of the cases. Mixed infections with both viruses and bacteria may occur in up to 45% of infections in children and 15% of infections in adults. A causative agent may not be isolated in approximately half of cases despite careful testing.

The term "pneumonia" is sometimes more broadly applied to any condition resulting in inflammation of the lungs (caused for example by autoimmune diseases, chemical burns or drug reactions); however, this inflammation is more accurately referred to as pneumonitis.

Conditions and risk factors that predispose to pneumonia include smoking, immunodeficiency, alcoholism, chronic obstructive pulmonary disease, asthma, chronic kidney disease, and liver disease. The use of acid-suppressing medications—such as proton-pump inhibitors or H2 blockers—is associated with an increased risk of pneumonia. The risk is also increased in old age.

Congenital toxoplasmosis is a specific form of toxoplasmosis in which an unborn fetus is infected via the placenta. Congenital toxoplasmosis is associated with fetal death and abortion, and in infants, it is associated with neurologic deficits, neurocognitive deficits, and chorioretinitis. A positive antibody titer indicates previous exposure and immunity, and largely ensures the unborn fetus' safety. A simple blood draw at the first prenatal doctor visit can determine whether or not a woman has had previous exposure and therefore whether or not she is at risk. If a woman receives her first exposure to "T. gondii" while pregnant, the fetus is at particular risk.

Not much evidence exists around the effect of education before pregnancy to prevent congenital toxoplasmosis. However educating parents before the baby is born has been suggested to be effective because it may improve food, personal and pet hygiene. More research is needed to find whether antenatal education can reduce congenital toxoplasmosis.

For pregnant women with negative antibody titers, indicating no previous exposure to "T. gondii", serology testing as frequent as monthly is advisable as treatment during pregnancy for those women exposed to "T. gondii" for the first time dramatically decreases the risk of passing the parasite to the fetus. Since a baby's immune system does not develop fully for the first year of life, and the resilient cysts that form throughout the body are very difficult to eradicate with antiprotozoans, an infection can be very serious in the young.

Despite these risks, pregnant women are not routinely screened for toxoplasmosis in most countries, for reasons of cost-effectiveness and the high number of false positives generated; Portugal, France, Austria, Uruguay, and Italy are notable exceptions, and some regional screening programmes operate in Germany, Switzerland and Belgium. As invasive prenatal testing incurs some risk to the fetus (18.5 pregnancy losses per toxoplasmosis case prevented), postnatal or neonatal screening is preferred. The exceptions are cases where fetal abnormalities are noted, and thus screening can be targeted.

Pregnant women should avoid handling raw meat, drinking raw milk (especially goat milk) and be advised to not eat raw or undercooked meat regardless of type. Because of the obvious relationship between "Toxoplasma" and cats it is also often advised to avoid exposure to cat feces, and refrain from gardening (cat feces are common in garden soil) or at least wear gloves when so engaged. Most cats are not actively shedding oocysts, since they get infected in the first six months of their life, when they shed oocysts for a short period of time (1–2 weeks.) However, these oocysts get buried in the soil, sporulate and remain infectious for periods ranging from several months to more than a year. Numerous studies have shown living in a household with a cat is not a significant risk factor for "T. gondii" infection, though living with several kittens has some significance.

In 2006, a Czech research team discovered women with high levels of toxoplasmosis antibodies were significantly more likely to have baby boys than baby girls. In most populations, the birth rate is around 51% boys, but women infected with "T. gondii" had up to a 72% chance of a boy. In mice, the sex ratio was higher in early latent toxoplasmosis and lower in later latent toxoplasmosis.

Bacteria are the most common cause of community-acquired pneumonia (CAP), with "Streptococcus pneumoniae" isolated in nearly 50% of cases. Other commonly isolated bacteria include "Haemophilus influenzae" in 20%, "Chlamydophila pneumoniae" in 13%, and "Mycoplasma pneumoniae" in 3% of cases; "Staphylococcus aureus"; "Moraxella catarrhalis"; "Legionella pneumophila" and Gram-negative bacilli. A number of drug-resistant versions of the above infections are becoming more common, including drug-resistant "Streptococcus pneumoniae" (DRSP) and methicillin-resistant Staphylococcus aureus (MRSA).

The spreading of organisms is facilitated when risk factors are present. Alcoholism is associated with "Streptococcus pneumoniae", anaerobic organisms, and "Mycobacterium tuberculosis"; smoking facilitates the effects of "Streptococcus pneumoniae", "Haemophilus influenzae", "Moraxella catarrhalis", and "Legionella pneumophila". Exposure to birds is associated with "Chlamydia psittaci"; farm animals with "Coxiella burnetti"; aspiration of stomach contents with anaerobic organisms; and cystic fibrosis with "Pseudomonas aeruginosa" and "Staphylococcus aureus". "Streptococcus pneumoniae" is more common in the winter, and should be suspected in persons aspirating a large amount of anaerobic organisms.

When a pregnant woman is diagnosed with acute toxoplasmosis, amniocentesis can be used to determine whether the fetus has been infected or not. When a pregnant woman develops acute toxoplasmosis, the tachyzoites have approximately a 30% chance of entering the placental tissue, and from there entering and infecting the fetus. As gestational age at the time of infection increases, the chance of fetal infection also increases.

If the parasite has not yet reached the fetus, spiramycin can help to prevent placental transmission. If the fetus has been infected, the pregnant woman can be treated with pyrimethamine and sulfadiazine, with folinic acid, after the first trimester. They are treated after the first trimester because pyrimethamine has an antifolate effect, and lack of folic acid can interfere with fetal brain formation and cause thrombocytopaenia. Infection in earlier gestational stages correlates with poorer fetal and neonatal outcomes, particularly when the infection is untreated.

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.

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

FLD affects approximately .5%-3% of farmers. In some regions of the world such as Asia, the infection rate is more around 6%.

It can be classified into acute interstitial pneumonitis, blood pneumonitis, lymphocytic interstitial pneumonitis, radiation pneumonitis, and uremic pneumonitis.

Whether aspiration pneumonia represents a true bacterial infection or a chemical inflammatory process remains the subject of significant controversy. Both causes may be present with similar symptoms.

BFL symptoms improve in the absence of the bird proteins which caused the disease. Therefore, it is advisable to remove all birds, bedding and pillows containing feathers from the house as well as washing all soft furnishings, walls, ceilings and furniture. Certain small mammals kept as pets have the same or similar proteins in their fur and feces and so should be removed. Peak flow measurements will indicate a lung condition however a spirometric test on lung capacity and patients ability to move air in and out of the lungs plus in more advanced cases an X-ray test or CT scan is available to confirm whether someone has the disease or not. Steroid inhalers similar to those used for asthma are effective or in cases where the patient finds inhaling difficult high dosages of steroids combined with bone density protecting drugs are used to treat a person with BFL, reducing the inflammation and hopefully preventing scarring. Recovery varies from patient to patient depending on what stage the condition was at when the patient consulted the doctor, the speed of diagnosis and application of the appropriate treatment to prevent residual damage to the lungs and many make a full recovery. However, BFL may reoccur when in contact with birds or other allergens.

Aspiration pneumonia is often caused by a defective swallowing mechanism, often due to a neurological disease or as the result of an injury that directly impairs swallowing or interferes with consciousness. Examples of the former are stroke, Parkinson's disease, and multiple sclerosis, and examples of the latter are some types of dementia, seizures, intoxication, and general anaesthesia. For many types of surgical operations, patients are therefore instructed to take nothing by mouth (nil per os, abbreviated as NPO) for at least four hours before surgery.

Bird fancier's lung is a type of hypersensitivity pneumonitis caused by bird droppings. The lungs become inflamed with granuloma formation.

Bird fancier's lung (BFL), also called "bird-breeder's lung" and "pigeon-breeder's lung", is a subset of hypersensitivity pneumonitis (HP). This disease is caused by the exposure to avian proteins present in the dry dust of the droppings and sometimes in the feathers of a variety of birds. Birds such as pigeons, parakeets, cockatiels, shell parakeets (budgerigars), parrots, turtle doves, turkeys and chickens have been implicated.

People who work with birds or own many birds are at risk. Bird hobbyists and pet store workers may also be at risk.

Pneumonitis or pulmonitis is an inflammation of lung tissue due to factors other than microorganisms. Those can be radiation therapy of the chest , exposure to medications used during chemo-therapy, the inhalation of debris (ie animal dander), of food particles during vomiting, herbicides or fluorocarbons and some systemic diseases.

It is distinguished from pneumonia on the basis of causation as well as its manifestation since pneumonia can be described as pneumonitis combined with consolidation and exudation of lung tissue due to infection with microorganism.

Hypersensitivity pneumonitis may also be called many different names, based on the provoking antigen. These include:

Of these types, Farmer's Lung and Bird-Breeder's Lung are the most common. "Studies document 8-540 cases per 100,000 persons per year for farmers and 6000-21,000 cases per 100,000 persons per year for pigeon breeders. High attack rates are documented in sporadic outbreaks. Prevalence varies by region, climate, and farming practices. HP affects 0.4–7% of the farming population. Reported prevalence among bird fanciers is estimated to be 20-20,000 cases per 100,000 persons at risk."

Endogenous lipoid pneumonia and non-specific interstitial pneumonitis has been seen prior to the development of pulmonary alveolar proteinosis in a child.

Sources of such lipids could be either exogenous or endogenous.

Exogenous: from outside the body. For example, inhaled nose drops with an oil base, or accidental inhalation of cosmetic oil. Amiodarone is an anti-arrythmic known to cause this condition. Oil pulling has also been shown to be a cause. At risk populations include the elderly, developmentally delayed or persons with gastroesophageal reflux. Switching to water-soluble alternatives may be helpful in some situations.

Endogenous: from the body itself, for example, when an airway is obstructed, it is often the case that distal to the obstruction, lipid-laden macrophages (foamy macrophages) and giant cells fill the lumen of the disconnected airspace.

Chemical pneumonitis is inflammation of the lung caused by aspirating or inhaling irritants. It is sometimes called a "chemical pneumonia", though it is not infectious. There are two general types of chemical pneumonitis: acute and chronic.

Irritants capable of causing chemical pneumonitis include vomitus, barium used in gastro-intestinal imaging, chlorine gas (among other pulmonary agents), ingested gasoline or other petroleum distillates, ingested or skin absorbed pesticides, gases from electroplating, smoke and others. It may also be caused by the use of inhalants.

Mendelson's syndrome is a type of chemical pneumonitis.

Mineral oil should not be given internally to young children, pets, or anyone with a cough, hiatus hernia, or nocturnal reflux, because it can cause complications such as lipoid pneumonia. Due to its low density, it is easily aspirated into the lungs, where it cannot be removed by the body. In children, if aspirated, the oil can work to prevent normal breathing, resulting in death of brain cells and permanent paralysis and/or retardation

Sixty percent of people with acute interstitial pneumonitis will die in the first six months of illness. The median survival is 1½ months.

However, most people who have one episode do not have a second. People who survive often recover lung function completely.

Acute interstitial pneumonitis occurs most frequently among people older than forty years old. It affects men and women equally. There are no known risk factors; in particular, smoking is not associated with increased risk.

Fire breathing is typically performed with a high flash point fuel, such as lamp oil (liquid paraffin), while fire eating is performed with low flash point fuels, such as white gas or naphtha. Highly purified fuels are preferred by fire performers due to their minimized toxicity, but other, more dangerous fuels may sometimes be used, such as ethanol, isopropanol, kerosene, gasoline, or charcoal lighter fluid. All fuels run the risk of causing pneumonitis if inhaled, however longer chain oils are more persistent than smaller molecules. Alcohols and volatile naphthas are likely to be absorbed or expelled from the body by evaporation and respiration.

The best treatment is to avoid the provoking allergen, as chronic exposure can cause permanent damage. Corticosteroids such as prednisolone may help to control symptoms but may produce side-effects.

The only prevention for FLD is ventilating the work areas putting workers at risk and using face masks to filter out the antigens attempting to enter the lungs through the air.

Possible causes of lymphocytic interstitial pneumonia include the Epstein-Barr virus and Human Immunodeficiency Virus.

Each type of vertically transmitted infection has a different prognosis. The stage of the pregnancy at the time of infection also can change the effect on the newborn.

Flock worker's lung is caused by exposure to small pieces of flock, usually nylon, created during the flocking process and inhaled. Exposure to rotary-cut flock particulates is the main risk factor; whether or not other types of flock cause this pulmonary fibrosis is not yet determined. Other types of flock include rayon, polypropylene, and polyethylene. Workers exposed to nylon, polypropylene, polyethylene, and rayon flocking debris have developed flock worker's lung. Exposure to higher concentrations of respirable flock particles is associated with more severe disease.

Whether or not smoking affects the progression or incidence of flock worker's lung is a topic of ongoing research as of 2015. Research in rats has shown that nylon flocking is a causative agent.