Many of the viruses, bacteria, parasites, and fungi that can invade the human body are also capable of attacking the surface or interior of the eye. Infectious eye diseases can be categorized in two ways.
Firstly, doctors normally talk about the part of the eye that's infected or inflamed. Conjunctivitis, for example, is an inflammation of the conjunctiva, the membrane of the inner eyelid and the inner corner of the eye's surface. Other possible locations of inflammation include the eyelid (blepharitis), the cornea (keratitis), the liquid inside the eye (vitritis), the retina and the blood vessels that feed it (chorioretinitis), or the optic nerve (neuroretinitis). These are just a few examples - the eye is a complex organ of many parts.
Secondly, eye infections are also classified according to what's causing them. Ocular histoplasmosis syndrome (OHS), for example, is caused by a fungus (the condition is also called chorioretinitis). It generally attacks the blood supply of the retina, on the inner rear surface of the eye.
The most common eye infection is conjunctivitis caused by an adenovirus (a type of common cold virus). This type of infectious conjunctivitis is sometimes called pinkeye and is most common in children. Viral conjunctivitis is contagious because the virus can be spread from the eye to hands that then touch doorknobs and other surfaces that other people use.
There are other causes of infectious conjunctivitis, such as bacteria like Staphylococcus aureus. Bacterial infections occur most commonly in children and tend to result in longer-lasting cases of pinkeye.