Chlamydia, kluh MIHD ee uh, is the common name for a group of bacteria that cause disease in animals and human beings. In the United States, genital chlamydia infection is the most common bacterial sexually transmitted disease (STD). It is caused by the bacterium Chlamydia trachomatis, which also causes another STD called lymphogranuloma venereum. Medical experts estimate that chlamydia strikes about 4 million Americans each year, mostly young people under 25 years of age. In men, symptoms of chlamydia appear about 7 to 21 days after infection. Most infected men develop a white or colorless discharge from the penis, accompanied by painful urination. The symptoms may go unnoticed, and they usually disappear after several months. Left untreated, the patient may infect other people.