Infectious conjunctivitis is most commonly caused by a viral infection. There are several viruses which can cause conjunctivitis, including adenoviruses (most common), herpes simplex virus, varicella zoster virus, and molluscum contagiosum virus. Another common cause is a bacterial infection, which is more common in children.
These germs usually get spread by touching the eyes after touching objects contaminated by the viruses or bacteria. In addition, bacteria can also be spread from one person to another through sexual contact or from a mother to her baby during childbirth.