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 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.

Most 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. 1, 2, 3, 4, 5, 6, 7

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
  • Nursing homes
  • Hospitals
  • Prisons

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


1 Csango PA, Haraldstad S, Pedersen JE, Jagars G, Foreland I. Respiratory tract infection due to Chlamydia pneumoniae in military personnel. Scand J Infect Dis Suppl. 1997;104:26–9.

2 Dawood FS, Ambrose JF, Russell BP et al. Outbreak of pneumonia in the setting of fatal pneumococcal meningitis among US Army trainees:  potential role of Chlamydia pneumoniae infection. BMC Infect Dis. 2011;11:157.

3 Oktem IM, Ellidokuz H, Sevinc C, et al. PCR and serology were effective for identifying Chlamydophila pneumoniae in a lower respiratory infection outbreak among military recruits. Jpn J Infect Dis. 2007;60:97–101.

4 Schmidt SM, Muller CE, Mahner B, Wiersbitzky SK. Prevalence, rate of persistence and respiratory tract symptoms of Chlamydia pneumoniae infection in 1211 kindergarten and school age children. Pediatr Infect Dis J. 2002;21:758–62.

Conklin L, Adjemian J, Loo J, et al. Investigation of a Chlamydia pneumoniae outbreak in a federal correctional facility in Texas. Clin Infect Dis. 2013;57:639–47.

Mylotte J. Nursing home-acquired pneumonia. Clin Infect Dis. 2002;35:1205–11.

Kuo C, Jackson LA, Campbell LA, Grayston JT. Chlamydia pneumoniae (TWAR). Clin Microbiol Rev. 1995;8:451–61.

Fajardo KA, Zorich SC, Voss JD, Thervil JW. Pneumonia outbreak caused by Chlamydophila pneumoniae among US air force academy cadets, Colorado, USA. EID. 2015;21.