Causes, How It Spreads, and People at Increased Risk

Chlamydia pneumoniae is a type of bacteria that causes respiratory tract infections, such as pneumonia (lung infection). The bacteria cause illness by damaging the lining of the respiratory tract including the throat, windpipe, and lungs. Some people may become infected and have mild or no symptoms.

How it spreads

People spread C. pneumoniae by coughing or sneezing, which creates small respiratory droplets that contain the bacteria. Other people then breathe in the droplets and bacteria. People can also get sick if they touch something with droplets from a sick person on it and then touch their mouth or nose.

People who spend a short amount of time with someone who has C. pneumoniae infection usually do not become ill. However, it is common for the bacteria to spread between people who live together. C. pneumoniae infections usually have long incubation periods (the time between breathing in the bacteria and developing symptoms). Symptoms usually begin 3 to 4 weeks after exposure.

People at increased risk

People of all ages can get sick from C. pneumoniae. It most commonly infects people for the first time when they are school-aged children or young adults. However, reinfection is most common in older adults.

People at increased risk include those who live or work in crowded settings where outbreaks most commonly occur8, such as:

  • Schools
  • College residence halls
  • Military barracks
  • Long-term care settings
  • Hospitals
  • Prisons

Older adults are at increased risk for severe disease caused by C. pneumoniae infection, including pneumonia.

Page last reviewed: November 15, 2021