Chlamydia (say "kluh-MID-ee-uh") is an infection caused by bacteria. In many cases it is spread through sexual contact. It's treated with antibiotics.
In men it can cause burning when you urinate and infection of the long, tightly coiled tube that lies behind each testicle and collects sperm (epididymis). In women it can cause infection of the cervix and a pelvic infection that can be serious.
Other types of chlamydia infection are not transmitted sexually. These types can cause a serious eye infection, especially in babies, and pneumonia.