Chlamydia is a type of bacteria. There are several different species of chlamydia. A number of strains within each species are responsible for a variety of diseases in birds, humans, and other mammals. Their most common appearance is as a sexually transmitted disease (STD) referred to as chlamydia or nongonococcal urethritis (NGU).

Chlamydia is one of the most common STDs in the US, especially among sexually active teens and young adults. Prevalence is highest in people 25 years old and younger.

Chlamydia can lead to Reiter’s syndrome, which is a triad of arthritis, conjunctivitis, and urethritis. It can also cause neonatal infections such as pneumonia or conjunctivitis when passed from an infected mother. It can also cause infections in the throat or rectum through oral or anal intercourse.

Other types of chlamydia can cause:

• Another less common STD known as lymphogranuloma venereum (LGV)
• An eye infection called trachoma that causes millions of cases of blindness in developing nations around the world
• A lung and upper respiratory infection call psittacosis