Often called "pink eye", it is an inflammation of the conjunctiva, the tissue that lines the inside of the eyelid that helps keep the eyelid and eyeball moist. Viruses, bacteria, irritating substances (shampoo, dirt, smoke, pool chlorine), sexually transmitted diseases (STDs), or allergens (substances that cause allergies) can all cause conjunctivitis. When this condition is caused by bacteria, viruses, or STDs, it can spread easily from person to person but is not a serious health risk if diagnosed promptly; allergic conjunctivitis is not contagious. (Ref: WebMD)