The condition known as pink eye or conjunctivitis is an infection of the eyelids and a protective layer of the eye called the conjunctiva. It can be caused by bacteria that 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, 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.
The most common form of conjunctivitis is bacterial. This is because the eye does not have the usual defense mechanism for destroying harmful bacteria, like those that 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 colonies manage to overwhelm the defenses of the conjunctiva, the result is the infection we know as pink eye.
The bacterial form of pink eye is indeed contagious, and approximately 50% of all the reported cases are bacterial. The treatment is usually some form of antibiotic eye drops and scrupulous hygiene practices until the condition clears up. While the bacterial form is still producing infected fluids, however, the possibility of infecting someone else through casual contact is still present. 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% of all cases. Antibiotic eye drops would have little effect on a person with viral conjunctivitis, but there are treatments available that help to neutralize the virus. This form is also very contagious, so the same hygienic protocols should be in place. Patients should avoid direct contact with others, 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 conjunctivitis, which accounts for the remaining 30% 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 that often accompany hay fever, for example, would be considered an allergen-based form of conjunctivitis. This form is not 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 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, people should use antibacterial wipes or disinfectant sprays to prevent the spread of infection. People should avoid touching the 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.