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 cases.
Mixed infections with both viruses and bacteria may occur in roughly 45% of infections in children and in 15% of infections in adults.
A causative agent may not be isolated in about 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.

Factors that predispose to pneumonia include smoking, immunodeficiency, alcoholism, chronic obstructive pulmonary disease, sickle cell disease (SCD), asthma, chronic kidney disease, liver disease, and old age.
Additional risks in children include not being breastfed, exposure to cigarette smoke and other air pollution, malnutrition, and poverty.
The use of acid-suppressing medications – such as proton-pump inhibitors or H2 blockers – is associated with an increased risk of pneumonia.
Approximately 10% of people who require mechanical ventilation develop ventilator-associated pneumonia, and people with a gastric feeding tube have an increased risk of developing aspiration pneumonia.
For people with certain variants of the FER gene, the risk of death is reduced in sepsis caused by pneumonia.
However, for those with TLR6 variants, the risk of getting Legionnaires' disease is increased.
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 cases. Mixed infections with both viruses and bacteria may occur in roughly 45% of infections in children and in 15% of infections in adults. A causative agent may not be isolated in about 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.
Factors that predispose to pneumonia include smoking, immunodeficiency, alcoholism, chronic obstructive pulmonary disease, sickle cell disease (SCD), asthma, chronic kidney disease, liver disease, and old age. Additional risks in children include not being breastfed, exposure to cigarette smoke and other air pollution, malnutrition, and poverty. The use of acid-suppressing medications – such as proton-pump inhibitors or H2 blockers – is associated with an increased risk of pneumonia. Approximately 10% of people who require mechanical ventilation develop ventilator-associated pneumonia, and people with a gastric feeding tube have an increased risk of developing aspiration pneumonia. For people with certain variants of the FER gene, the risk of death is reduced in sepsis caused by pneumonia. However, for those with TLR6 variants, the risk of getting Legionnaires' disease is increased.
Pneumonia