The condition known as pink eye or conjunctivitis is an infection of the eyelids and a protective layer of the eye called the conjunctiva. Pink eye can be caused by bacteria which naturally reside in the eyelid, viruses which find their way to the eye area, or natural allergens which trigger an allergic response.

Of these three main causes of pink eye, only bacterial or viral infections are actually considered to be contagious. Conjunctivitis caused by hay fever or other allergic reaction is not generally contagious, but it is not always easy to tell the difference between the three forms of pink eye.

The most common form of pink eye or conjunctivitis is bacterial. This is because the eye does not have the usual defense mechanism for destroying harmful bacteria, like the bacteria which reside in the roots of eyelashes and along the rims of the eyelids themselves. A natural chemical present in the conjunctiva is supposed to neutralize bacteria, but it is not always successful. When bacteria colonies manage to overwhelm the defenses of the conjunctiva, the result is the infection we know as pink eye or conjunctivitis.

The bacterial form of pink eye is indeed contagious, and approximately 50 percent of all the reported cases of pink eye are bacterial. The treatment for bacterial pink eye is usually some form of antibiotic eye drops and scrupulous hygiene practices until the condition clears up. While the bacterial form of pink eye is still producing infected fluids, however, the possibility of infecting someone else through casual contact is still there. A classroom full of students or an office full of co-workers could easily be exposed through casual contact with an infected person.

Pink eye can also be triggered by a virus, but this form only accounts for 20 percent of all cases. Antibiotic eye drops would have little effect on a person with viral conjunctivitis, but there are treatments available which help to neutralize the virus. Viral pink eye is also very contagious, so the same hygienic protocols would be in place. Direct contact with others should be avoided, and any medical waste products such as bandages, tissues and eye droppers should be disposed of properly.

Allergens such as pollen and pollutants such as house dust can trigger a third form of pink eye which accounts for the remaining 30 percent of all cases. Treatment of allergen or pollutant-based pink eye is generally part of a larger treatment for the underlying allergy or reaction. The itchy, swollen eyes which often accompany hay fever, for example, would be considered an allergen-based form of conjunctivitis. This form of pink eye is not considered to be contagious, since the excess fluids do not contain either bacteria or viruses, only natural lachrymal fluids and flushed-out irritants.

Although nearly a third of all cases of pink eye are not considered contagious, it is not easy for an average person to distinguish contagious from non-contagious conjunctivitis. If a schoolmate or co-worker appears to have some form of an active eye infection, it is still best to err on the side of safety and avoid any casual contact or possible transfers from handling communal objects like telephones or toys. When in doubt, use antibacterial wipes or disinfectant sprays to prevent the spread of infection. Avoid touching your face or eyes after coming in contact with anyone who shows signs of an eye infection, with or without accompanying symptoms of a cold or flu.