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Pneumonia can be due to a variety of bacteria and viruses, less commonly due to fungi, and rarely due to other microorganisms or parasites. Likely causes are associated with the age of the person, the season, the affected person's health status, and where the infection was contracted.

Some examples of the causes of pneumonia are listed below:

Bacterial | Viral | Fungal | Unusual but serious

Bacterial causes 
Bacteria are the most common cause of pneumonia in adults but are uncommon in young children. There are many different types of bacteria that can cause pneumonia, some more common than others. Sometimes people, particularly those who are elderly or who have other health problems, will develop bacterial pneumonia after having a viral respiratory infection.

Some bacterial pneumonias are relatively rare but may be seen in certain populations or in certain geographic areas. Travelers and people who have immigrated from certain countries may become infected with bacteria that are less common in the U.S. Also, those with pre-existing conditions such as decreased lung function or weakened immune systems may be more vulnerable to types of bacteria not found in the general population.

The most common cause of bacterial pneumonia contracted outside of a hospital or healthcare setting (community-acquired) is:

  • Streptococcus pneumoniae—these bacteria cause pneumococcal pneumonia and are easily spread through normal social contact; a vaccine is available to help prevent pneumonia and other serious infections, including meningitis and blood infections (septicemia

Other less common causes include:

  • Haemophilus influenzae type B (Hib)—most often causes pneumonia but can cause other invasive infections such as meningitis and septicemia; however, infections are now less common due to Hib vaccinations
  • Moraxella catarrhalis—more typically a source of middle ear infections in children
  • Staphylococcus aureus—has become more common in hospital settings recently from drug-resistant strains like methicillin-resistant S. aureus (MRSA) or following an influenza infection
  • Klebsiella pneumoniae— can cause pneumonia in people already weakened by disease, usually in a hospital setting
  • Streptococcus agalactiae (Group B strep)—typically colonizes the female vagina and can be passed from a pregnant woman to her baby during childbirth
  • Pseudomonas aeruginosa—often affects those with decreased lung function due to cystic fibrosis

Atypical pneumonias are bacterial lung infections that do not respond to commonly prescribed antibiotics. Examples include:

  • Mycoplasma pneumoniae—a common cause of upper respiratory infections, with an estimated 2 million cases in the U.S. each year; responsible for 15-20% of cases of community-acquired pneumonia. It causes a less severe infection known as "walking pneumonia."
  • Chlamydophila pneumoniae—causes about 10% of the community acquired forms of pneuomonia
  • Legionella pneumophila—causes Legionnaires' disease; infections occur when an individual inhales steam, mist, or airborne droplets contaminated with the bacteria. The infection is not spread from one person to another. A legionella infection often requires hospitalization and has a significant mortality rate.

Mycobacterium tuberculosis and nontuberculous Mycobacteria (M. avium-intracellulare, M. kansasii) are rare in the U.S., but rates of infection are rising in other countries. Cases in the U.S. may be seen, for example, in travelers, prison populations, and those with AIDS.

Viral causes 
Viruses that infect the respiratory system cause about one-third of all cases of pneumonia in the U.S. each year and are the most common cause of pneumonia in infants, accounting for approximately 90% of all lower respiratory infections. Less than 20% of pneumonias in adults younger than 60 years old are viral. In those over 60-65 years of age, viral causes become increasingly common.

Viral pneumonia is often mild, requiring only supportive care. However, more serious cases may require hospitalization. Sometimes secondary bacterial pneumonia may develop following a primary viral infection.

The most common causes of viral pneumonia are:

  • Influenza virus—the most common cause of viral pneumonia in adults
  • Parainfluenza virus—commonly causes respiratory infections in infants and young children and can progress to pneumonia
  • Respiratory syncytial virus (RSV)—the most common cause in children younger than one year old

Other less common causes include:

  • Adenoviruses—common viruses that often cause respiratory illness but usually are not serious
  • Human metapneumovirus—this virus was first identified in 2001; in children, it is thought to cause about 5%-15% of hospitalizations due to lung infections.
  • Cytomegalovirus (CMV)—may cause serious physical and developmental problems when women are infected for the first time (primary infection) during pregnancy and then pass the infection to the developing baby across the placenta.
  • Measles—also called rubeola, is an extremely contagious viral infection that is transmitted through respiratory secretions. Most people recover within a couple of weeks, but up to 20% develop complications that may include pneumonia.
  • Varicella (chickenpox)—caused by an infection with the varicella zoster virus (VZV), a member of the herpes virus family; most cases resolve without complications. Pneumonia is an uncommon complication.
  • Rhinoviruses—the virus causing the common cold in children and adults; may occasionally infect the lower respiratory tract and cause pneumonia in young children and the elderly

Fungal causes 
Pneumonias caused by fungi are relatively rare. Some fungi cause pneumonia only in people who have weakened immune systems. These people include organ transplant recipients who are on immunosuppressant drugs, people on chemotherapy regimens, and those with AIDS. These fungi are referred to as "opportunistic." Examples include:

  • Pneumocystis jiroveci—the most common in the U.S. in people with AIDS; it was previously known as P. carinii; thus pneumonia caused by this organism may still sometimes called Pneumocystis carinii pneumonia or PCP.
  • Aspergillus species—can cause aspergillosis and fungal masses in the sinuses and lungs
  • Candida species—these yeast are part of the normal human flora; infections can occur in the moist mucous membranes of the body.
  • Cryptococcus neoformans—anyone may become infected, but the highest prevalence in the U.S. is in people who have HIV/AIDS

Some fungi are pathogenic meaning they can cause pneumonia in people regardless of their health status. People become infected when they breathe in fungal spores. These types are usually found only in certain areas of the U.S. and/or the world:

  • Histoplasma capsulatum—causes histoplasmosis and is found in soil contaminated with bat or bird droppings; typically found in Ohio and Mississippi river valleys
  • Coccidioides immitis— causes coccidioidomycosis, also called Valley Fever; often found in Arizona and parts of California
  • Blastomyces dermatididis— found in south central, southeastern, and midwestern U.S. in moist soil and decaying wood or leaves

For more on these, read the article on Fungal Infections and Fungal Tests.

Unusual and serious causes 
Some exotic and rare causes of pneumonia may infect otherwise healthy individuals and can be fatal. Examples include:

  • Hantavirus—contracted from infected rodent droppings in the southwestern U.S. In 2012, the U.S. National Park Service reported that ten people who stayed in Yosemite National Park, California were infected with hantavirus. Of those ten, three died and the other seven recovered.
  • The bacteria Bacillus anthracis (anthrax)—very rarely due to exposure to a natural source such as infected animals or animal products
  • The bacteria Yersinia pestis (pneumonic plague) may occur in the western U.S. due to breathing in droplets coughed up by a person infected with plague or when bubonic or septicemic plague spreads to the lungs
  • Middle East Respiratory Syndrome Coronavirus (MERS-CoV)–this newly discovered (2012) respiratory virus has caused serious lower respiratory infections in some people who live in or traveled to countries in the Arabian Peninsula. About one-third of infected individuals have died from the disease. As of 2014, however, there is no evidence that the virus spreads easily within communities and most cases of person-to-person spread have occurred in healthcare workers and other close contacts, such as family members and caregivers of people sick with MERS.

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