Viruses are the most common cause of contagious conjunctivitis, but it is often also caused by bacteria. If certain viruses or bacteria enter the eye, conjunctivitis can develop. It can also develop if a person has a virus or bacterial infection that works its way through the body to the eyes. The viral and bacterial forms are very contagious and spread extremely easily.
Allergic conjunctivitis is not contagious, since it’s not a viral or bacterial infection. This form is very common and develops when allergens irritate the eyes. The casual name pink eye typically only refers to the contagious forms.