Chlamydia, or chlamydial infection, is a sexually transmitted infection (STI) caused by Chlamydia trachomatis (C. trachomatis), a bacterium that only infects humans. Although we usually think of the sexually transmitted disease (STD), chlamydial infection refers to infection caused by any species of the Chlamydiaceae bacterial family. Chlamydia is a common infectious cause of genital and eye diseases in humans. It is the leading bacterial STI worldwide.

What is the difference between an STI and an STD?
A sexually transmitted infection (STI) is the infection, the entering of the bacterium, virus, or parasite into the human body (host). When symptoms appear it is called a sexually transmitted disease (STD).

What are sexually transmitted infections?
Sexually transmitted infections (STIs) are infections that spread primarily through person-to-person sexual contact. The word “primarily” is used here because sexual contact is the main or original source of infection and the main means of transmission; however, in some cases the infected person may pass the infection on without sexual contact, as might be the case when giving birth (mother infects baby), or if an infected person donates blood which is later used in a blood transfusion and infects a patient in hospital