Chlamydia is the most commonly reported STD in the U.S. It's spread mostly by vaginal or anal sex, but you can get it through oral sex, too. Sometimes you'll notice an odd discharge from your vagina or penis, or pain or burning when you pee. But only about 25% of women and 50% of men get symptoms.
Chlamydia is caused by bacteria, so it's treated with antibiotics. After you are treated, you should get retested in three months, even if your partner has been treated as well.