Chlamydia are germs (bacteria) which infect the genitals and sometime the eyes, throat and internal organs. It is an extremely clever germ which has learned to live inside human cells, for example those lining the genital tract or on the surface of the eye. Once Chlamydia are inside cells they are hidden from the body's normal mechanisms of resistance. They also can take advantage of all the nutrients that the host cell produces. We talk about Chlamydia with a capital C and put it in italics because, just like Pelargonium, it's the scientific name of a whole group of organisms.
Relatives of Chlamydia infect a huge range of animals from the humble amoeba to humans.
Chlamydia are able to cause long term, low key infections in the absence of treatment. If infection is allowed to grumble on in this way, there is a much greater chance of complications. Hence it is important do not delay in seeking treatment. It is better to be safe than sorry
For further information on the biology of chlamydiae see the Cell biology index on the professional site.