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. They have immunity, just as you develop immunity to Chicken Pox if you catch it. But even monitors can get sick when they have a wound. Just as a cut on the hand can get infected, a monitor lizard can get an infection. The first birds appeared during the Jurassic Period.