It may involve one or both eyes. The conjunctivae become red and swollen. With viral conjunctivitis there is a watery discharge. With bacterial conjunctivitis the discharge may become purulent (yellow pus) and the eyelashes stick together. Allergic conjunctivitis is very itchy. Sudden onset of pain and redness of the conjunctiva in one eye suggests a foreign body.
Very localised swelling of an eyelid is usually due to a stye (infection of a hair follicle). Local antibiotics, removal of the eyelash and warm compresses to open the obstructed hair follicle are needed.