When the gland becomes infected, the reptile can lose its sight. A blind reptile will probably have difficulty finding food or escaping from a dangerous predator. If reptiles don' t get the right kind of food, they may suffer diseases in their bones. A tick called Ophionyssus natricis sucks the blood of snakes. Sometimes the tick gives the snake a parasite that lives in the snake's blood.
Parasites also cause tumors in the muscles of reptiles.
Many reptiles don't seem to get sick unless they are injured. Some of them can heal their wounds very quickly. For example, monitor lizards do not seem to get fatal diseases. Monitors eat carrion, or the rotting meat of dead animals. Maybe this food source gives them the protection from disease.