The most common causes of pink eye include viruses, such as the common cold, bacteria, allergens and chemicals. Pink eye tends to have the same symptoms regardless of the cause, making it difficult to figure out what caused it in an individual.
Pink eye is a name for conjunctivitis, which is a condition that causes inflammation of the conjunctiva. A pink appearance is the most common symptom for all types of pink eye.
Viral conjunctivitis is highly contagious. Although it starts in one eye, it typically infects the other eye as well. Besides the pink appearance, a watery discharge in the affected eye is another common symptom. This type of pink eye clears up on its own within about a week.
Bacterial conjunctivitis is contagious, and a pus type discharge is common. This type of pink eye requires antibiotics, otherwise it may cause permanent damage to the eye.
Allergic conjunctivitis isn't contagious, as it's caused by that person's specific allergy to a substance, such as pollen or dust mites. It usually causes itchy eyes. The affected person must avoid the allergen to clear up the pink eye.
Chemical conjunctivitis isn't contagious, as it's also caused by a specific substance, such as smoke or shampoo. The pink eye clears up when the person avoids the chemical that caused it.