There are three main causes of conjunctivitis. One cause involves the introduction of either bacterial or viral microorganisms into the eye. It is not uncommon for a minor conjunctivitis to accompany a viral cold or flu. Both types of infection are contagious.
Irritants are another cause of conjunctivitis.
Offenders of this type include air pollutants, smoke, soap, hairspray, makeup, chlorine, cleaning fluids, etc. Lastly, some individuals acquire conjunctivitis due to a seasonal allergic response to grass and other pollens. Duration It usually takes from a few days to 2 weeks for most types of conjunctivitis to clear. Conjunctivitis due to an allergy may continue as long as the offending pollen is present. With such conditions, symptoms are likely to recur each year.