The condition known as conjunctivitis either conjunctivitis is an infection of the eyelids and a protective layer of the eye called conjunctiva . It can be caused by bacteria that naturally reside on the eyelid, viruses that reach the eye area, or natural allergens that trigger an allergic response. Of these three main causes, only bacterial or viral infections are considered truly contagious. Conjunctivitis caused by hay fever or other allergic reactions is usually not contagious, but it's not always easy to tell the three forms apart.
The most common form of conjunctivitis is bacterial. This is because the eye does not have the usual defense mechanism to destroy harmful bacteria, such as those that reside at the roots of the eyelashes and along the edges of the eyelids. A natural chemical present in the conjunctiva is supposed to neutralize bacteria, but it's not always successful. When the colonies manage to overwhelm the defenses of the conjunctiva, the result is the infection we know as conjunctivitis.
The bacterial form of conjunctivitis is actually contagious, and about 50% of all reported cases are bacterial. Treatment is usually some form of antibiotic eye drops and scrupulous hygiene practices until the condition clears. While the bacterial form still produces infected fluids, the possibility of infecting another person 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.
Conjunctivitis can also be caused by a virus, but this form only accounts for 20% of all cases. Antibiotic eye drops would have little effect on someone with viral conjunctivitis, but there are treatments available that help neutralize the virus. This form is also very contagious, so the same hygiene protocols must be in place. Patients should avoid direct contact with other people, and any medical waste products such as bandages, tissues, and droppers should be disposed of properly.
Allergens like pollen and pollutants like house dust can trigger a third form of conjunctivitis, which accounts for the remaining 30% of all cases. Treatment of allergen- or contaminant-based conjunctivitis is usually part of a larger treatment for the underlying allergy or reaction. The itchy, puffy 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 bacteria or viruses, only natural tear fluids and eliminated irritants.
Although almost a third of all cases are not considered contagious, it is not easy for the average person to distinguish contagious from non-contagious conjunctivitis. If a schoolmate or coworker appears to have some form of active eye infection, it's still best to err on the side of safety and avoid any casual contact or potential transfer when handling common objects like phones or toys. When in doubt, people should use antibacterial wipes or disinfectant sprays to prevent the spread of infection. People should avoid touching their face or eyes after coming into contact with anyone who shows signs of an eye infection, with or without cold or flu symptoms.