We didn't always know that Chlamydia trachomatis, a bacterial pathogen, causes chlamydia, nor did we know it's a sexually transmitted infection.
Chlamydiae are microorganisms, and they're unique because they have both viral and bacterial properties. Like a virus, they need a host cell in which to grow — although initially because the microorganism relies upon a host, it was first believed to be a protozoan parasite. Neither a parasite nor a virus, chlamydiae, just like gram-negative bacteria, have DNA and RNA, replicate through binary fission and have cell walls with an outer membrane.
There are multiple strains of C. trachomatis, and not all of those strains cause genital tract infections. Through hand-to-eye contact, for instance, some strains of C. trachomatis may cause respiratory and eye infections, such as severe chlamydial conjunctivitis, neonatal conjunctivitis or a chronic infection called trachoma. As long ago as 15 B.C.E., both ancient Egyptian and Chinese writings describe a chlamydialike infection among diseases of the eye, not the genitals, and it wasn't until 1976 that chlamydia was recognized as an STD [source: CDC, Black].
Today, chlamydia is one of the most common bacterial infections and the most frequently reported STD in the U.S. Nearly 1.5 million cases of genitourinary chlamydia infections were reported in 2013. But because up to half of infected males are asymptomatic, and as many as 70 percent of women don't experience symptoms, health professionals suggest the actual number of people infected is closer to 3 million [sources: HRF, Black]. When symptoms are present they commonly include abnormal discharge and painful urination. Chlamydia is treatable with antibiotics; left untreated, it may cause long-term complications such as infertility and chronic pelvic pain.