Chlamydiae are obligate intracellular gram-negative bacteria that have a unique two-stage cycle.

i) The infectious and nonreplicating “elementary body”, representing the extracellular stage, is a 0.2- to 0.4 micrometer sphere that has a rigid cell envelope and is metabolically inactive.

ii) The intracytoplasmic second stage, the reticulate body, is larger - 0.7 to 1.0 micrometer in diameter- and metabolically active, and multiplies by binary fission.

Chlamydial infections are widespread among birds and mammals, and perhaps 20% of the human population is infected.

Human diseases caused by chlamydiae can be divided into two types: (1) chlamydial agents transmitted by direct contact (C trachomatis genital and ocular infections, C pneumoniae ocular infection) and (2) chlamydial agents that are transmitted by the respiratory route (C psittaci and C pneumoniae).

Chlamydial diseases in humans include:

Chlamydial Conjunctivitis (Inclusion Conjunctivitis) ;

Trachoma ;

Chlamydial Infection of the Genital Tract ;

Psittacosis (Ornithosis, Parrot Fever) ;

Lymphogranuloma Venereum ;