History and Disease Patterns


In the 1960s, researchers discovered Chlamydia pneumoniae during vaccine studies to prevent a bacterial eye infection called trachoma. Researchers first thought C. pneumoniae were a virus. Now experts know that C. pneumoniae are bacteria and are not associated with eye infections. In 1983, scientists isolated the bacteria from a respiratory culture for the first time. This development helped scientists understand how C. pneumoniae cause illness in people.

Scientists classify C. pneumoniae as “atypical” bacteria for several reasons. Signs and symptoms of C. pneumoniae pneumonia (lung infection) are different from that of “typical” pneumonia caused by bacteria such as Streptococcus pneumoniae. In addition, antibiotics usually used to treat “typical” pneumonia may not work against C. pneumoniae infections. Also, C. pneumoniae are not able to be detected by some traditional microbiology methods.

Disease patterns

Experts do not know how many people get a C. pneumoniae infection each year. It is likely that many C. pneumoniae infections are not identified due to it usually only causing mild or no symptoms. People can get sick any time of the year; there isn’t a particular season when people are more likely to get sick from a C. pneumoniae infection. Outbreaks occur mostly in crowded environments like college residence halls and long-term care settings.

Page last reviewed: November 15, 2021