There are several things that can cause “Pink Eye.”

Infection by virus (20-50% of all cases)
Infection by bacteria (40–60 % of all cases)
Allergic reaction (10-20 % of all cases)
Trauma, chemical injuries (2 -5 % of all cases)

Viral conjunctivitis is one of the major causes of "Pink Eye" and Adenovirus represents close to 90% of these viral cases.