Chlamydia are bacteria, but they are not capable of surviving on their own. They can only grow inside other living cells, like viruses. Outside of living cells, chlamydia are dormant, like spores. In their dormant form, they can travel from one person or animal to another.

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 genital infection referred to as chlamydia or nongonococcal urethritis (NGU).

Chlamydia is one of the most common sexually transmitted disease (STD) in the US, especially among sexually active teens and young adults. Over a million cases were reported to the Centers for Disease Control and Prevention (CDC) in 2006.

This strain also causes Reiter’s syndrome (arthritis, conjunctivitis , and urethritis ). It can also cause neonatal infections— pneumonia or conjunctivitis (pink eye)—when passed from an infected mother.

Other types of chlamydia can cause:

  • Another less common STD known as lymphogranuloma venereum (LGV)
  • An eye infection called trachoma or Egyptian ophthalmia that causes millions of cases of blindness in developing nations around the world—This infection is known in developed countries as inclusion conjunctivitis or inclusion blenorrhea
  • Lung, heart, and intestinal infections


Genital chlamydial infections are caused when Chlamydia trachomatis is transmitted during oral, vaginal, or anal sex from an infected partner. Other forms of chlamydia can be transmitted by nonsexual contact, such as flies, dirty hands, or other objects, as well as inhalation and childbirth.

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