Real Dictionary

chlamydia
uncountable noun: Chlamydia is a sexually-transmitted disease.

chlamydia in American English
a widespread, gonorrhealike venereal disease caused by a bacterium (Chlamydia trachomatis) that also causes trachoma, etc.
noun: any coccoid rickettsia of the genus Chlamydia, parasitic in birds and mammals, including humans, and causing various infections, esp. of the eyes, as trachoma, lungs, as psittacosis, and genitourinary tract, as urethritis or chlamydia

chlamydia in British English
noun: any Gram-negative bacteria of the genus Chlamydia, which are obligate intracellular parasites and are responsible for such diseases as trachoma, psittacosis, and some sexually transmitted diseases