Chlamydia (say "kluh-MID-ee-uh") is an infection caused by bacteria. In many cases, it's spread through sexual contact (sexually transmitted). It's treated with antibiotics.
Chlamydia often doesn't cause symptoms. When it does, they may include burning when you urinate, pain during sexual intercourse, or abnormal discharge from the vagina or penis. If chlamydia isn't treated, it can lead to a severe infection of the uterus, fallopian tubes, or ovaries. (This is called pelvic inflammatory disease.)
Some types of chlamydia infection aren't spread through sexual contact. These types can cause a serious eye infection, especially in babies.