Eyelid infections generally involve the lid margins, eyelid glands or follicles causing styes or hordeolums. They are usually associated with Staphylococcus aureus infections. The conjunctiva is particularly susceptible to infection. The epithelial surface enclosed by the eyelids is a warm moist enclosed environment in which microorganisms can rapidly become established. However, microorganisms must avoid being rinsed away by tears and some pathogens, such as Chlamydia trachomatis, attach specifically to conjunctival cells.

An estimated 500 million people are infected with different serotypes of Chlamydia trachomatis, making trachoma the most significant eye infection worldwide. The disease blinds approximately 1% of infected individuals while many others suffer visual impairment. Chlamydia trachomatis is transmitted by contact with contaminated flies, fingers and towels although trachoma itself results from chronic repeated infections. This is much more likely to occur in regions where restricted access to water prevents regular washing of the hands and face. Chlamydia is also a sexually transmitted disease and there is evidence that untreated chlamydial infections can lead to premature delivery and babies born to infected mothers can be infected in their eyes and respiratory tracts. Chlamydia is a leading cause of early infant pneumonia and conjunctivitis (pink eye) in the newborn.

A laboratory diagnosis of trachoma can be carried out using samples of conjunctival fluid or scrapings. The usual treatment is with oral or topical antibiotics, such as tetracycline or doxycycline. Chlamydia infections account for only a fifth of cases of conjunctivitis; others are caused by bacteria such as Streptococcus pneumoniae and Leptospira spp.