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.
Infectious conjunctivitis is the most common cause of pinkeye around the world. Causes of infectious conjunctivitis are numerous and can usually be classified as viral, bacterial, or fungal.
Some of the most common causes of serious eye infection include:
Ocular histoplasmosis syndrome (OHS): Histoplasmosis is a fungal infection of the lungs, which is caught by inhaling spores. It's common in river valleys around the world. It's far more common in the US than in Canada, and is concentrated in the area known as the Bible Belt (called the "Histo Belt" by eye doctors). Over 90% of adults in the southeastern US have had histoplasmosis, which usually causes no symptoms. In a small fraction of cases, the fungus migrates to the retina many years or decades later. Once there, it damages the retina, particularly the macula (the vital centre part where the vision cells are most concentrated). It causes symptoms and retinal decay very similar to macular degeneration, and can destroy the central part of the field of vision. People of African descent are largely immune. Although only a tiny minority of people with histoplasmosis go on to suffer OHS, the fungus is so common that OHS is a significant infectious cause of legal blindness in Americans between the ages of 20 to 40 years.
Chlamydia and gonorrhea: These extremely common sexually transmitted infections (STIs) can cause conjunctivitis, though they don't usually cause serious damage to the eye in adults. The infection gets into the eye either directly through genital fluids such as semen, or when infected people rub their eyes after handling infected genital areas. Babies born to genitally infected women are at especially high risk of eye infection. Neisseria gonorrheae is one of the few bacteria capable of penetrating the protective layers of the eye, causing inner-eye infection.
Herpes simplex: This widely prevalent virus can be caught as a skin disease (cold sores) or as an STI. Herpes viruses can infect the eye in the same way as chlamydia or gonorrhea. Like these diseases, herpes can cause pitting and ulceration of the cornea. Chronic herpes infection, which is uncommon, can cause acute retinal necrosis (ARN), particularly in men. This causes a major destruction of retinal tissue, and causes dramatic damage to vision. About 15% of people with chronic ocular herpes simplex lose some vision.
Shingles (herpes zoster, varicella zoster): Shingles are a reactivation of the virus that initially causes chickenpox. The sores known as shingles are infectious and can cause chickenpox in others. They can also cause ocular infection if you touch the eyes after touching a sore. While herpes simplex is the leading cause of acute retinal necrosis in the young, varicella zoster is the leading cause in people over 50 years of age because shingles is more common in this age group.
Bacterial keratitis: This is an infection of the cornea by common bacteria found on the skin and in the mouth and nose. Normally, these bacteria can't penetrate the outer layer of the eye, and cause only conjunctivitis. However, eye injury, lack of oxygen due to contact lenses, or a weak immune system can all facilitate entry into the cornea, the clear layer in the front of the eye. Fungi can cause fungal keratitis under similar circumstances.
Infections that can cause conjunctivitis or keratitis (inflammation of the cornea) include:
Infections that can damage the retina and the inner eye include:
Obviously, with so many possible causes, the symptoms of eye infection can vary a lot. What a person feels generally depends more on where the infection is rather than on what's causing it.
Common symptoms of conjunctivitis include:
Common symptoms of keratitis and other frontal eye infections are:
Unfortunately, diseases that damage the retina, the optic nerve, or the blood vessels that feed them often cause no pain at all. The primary symptom is deteriorating vision, which is usually stoppable but not reversible. That's why it's vital to get your eyes checked regularly. One possible symptom of internal eye damage is floaters, tiny fragments in the liquid inside the eye. You see tiny bubbles or dark spots slowly falling through your line of vision. Everyone has a few floaters - you should only worry if you notice a sudden increase in them.
Almost all eye infections accompany disease in some other part of the body, even if it's just a cold. Some but not all of these diseases have clear symptoms. Be on the lookout for eye pain or visual symptoms if you have any of the diseases listed in the "Causes" section of this article.
Serious complications of eye infection include damage to the retina and the formation of scars and ulcers in the cornea that can obstruct vision. Some infections, like syphilis, can also provoke glaucoma. Moreover, eye problems are often the only visible symptom of wider infections. Chlamydia, for example, often causes no genital symptoms, but can cause infertility and heart damage if left untreated.