You can make a good guess about what's causing eye infections by looking to see if one of both eyes are infected. If just one eye is swollen of has a green or yellow discharge, it's probably a result of an injury from a poke with a stick, a fight with another animal, or even an embedded grass seed. If both eyes are infected, it's probably because of a virus. Eye infections are easy to treat, but they can cause blindness if they aren't treated quickly and correctly. See a veterinarian as soon as you notice symptoms.