5. Bacterial infections.
Like viruses, there are several types of bacteria that can affect the eye and cause infectious keratitis. Often, individuals who wear contact lenses are affected by keratitis that spreads from bacterial infections, as the bacteria can attach to a contact, which can then enter the eye; however, individuals who do not wear contact lenses can also develop this type of keratitis.
Two of the most common types of bacteria that cause keratitis include pseudomonas aeruginosa and staphylococcus aureus. The former is an extremely common bacteria that can be found throughout virtually all environments. It weakens the immune system and creates an infection; it can also produce toxins that damage tissues.
The latter is also extremely common and is often found in the nose or on the skin; it is responsible for causing staph infections, which can become impact various parts of the body, including the eyes.