Eye infections can be caused by viral, bacterial, and fungal microorganisms. These organisms do not cause infections solely in the eye. In reality, eye infections tend to occur as infections disseminate, or spread, in the body.

Microbiological infections of the eye involve the conjunctiva, which is the membrane of the inner eyelid and corner of the eye. This infection is termed conjunctivitis. Depending on the microbial agent, infection can also occur on the eyelid (blepharitis), the cornea (keratitis), the retina and its associated blood vessels (chorioretinitis), the optic nerve (neuroritinitis), and even the fluid inside the eyeball (vitritis).

A virus associated with eye infections is the Herpes Zoster virus. This virus is the reactivated form of the chickenpox virus that had previously established an infection, often in childhood. A hallmark of reactivation is the dissemination of the virus throughout the body via nerve fibers. The eye can become infected through the optic nerve fibers.

Typically, the viral infection will be a rash or inflammation on the upper or lower eyelid and the conjunctiva. The inside of the eye and the optic nerve leading from the retina to the brain can also become infected. Herpes Zoster eye infections can produce redness, swelling, pain, light sensitivity, and blurred vision.

The cornea of the eye is prone to infection by the type of fungi known as molds, and by yeast. Such an infection is termed mycotic keratitis. Infections can arise following eye surgery, from the use of contaminated contact lens (or the contamination of the contact lens cleaning solution), or due to a malfunction of the immune system. A common fungal cause of eye infections are species of Aspergillus. A common yeast source of infection are species of Candida. The eye infection may be a secondary result of the spread of a fungal or yeast infection elsewhere in the body. For example, those afflicted with acquired immunodeficiency syndrome can develop eye infections in addition to other fungal or yeast maladies.

Bacterial eye infections are often caused by Chlamydia, Neiserria, and Pseudomonas. The latter bacteria, which can infect the fluid used to clean contact lenses, can cause the rapid development of an infection that can so severe that blindness can result. Removal of the infected eye is sometimes necessary to stop the infection.

Less drastic solutions to infections include the use of antimicrobial eye drops.