Pink eye is usually caused by a viral or bacterial infection. It can also be caused by allergies, by exposure to chemical irritants, or rarely, by an underlying autoimmune disease such as rheumatoid arthritis or systemic lupus erythematosus.  

Infectious pink eye (caused by a virus or bacterium) may affect one or both eyes and is usually associated with an upper respiratory infection such as the common cold, a sinus infection, or a sore throat. Both adults and children can get the infectious type of pink eye from viruses or bacteria, but pink eye caused by bacterial infections is more common among children than adults. Both the viral and bacterial types tend to be highly contagious.

Whether the inflammation is caused by an infection, allergies, or another factor, the result is the same. The inflammation causes small blood vessels in the conjunctiva to dilate and become more visible, making the white of the eye appear pink or red.

Young infants can get recurrent pink eye if they have a blockage in the tear ducts.