Animals have three ways to protect themselves from disease; avoidance, resistance, and tolerance. Avoidance is exactly what it sounds like: I, and many other animals, will avoid meat that smells like death, for example. Resistance is run by the immune system: if I get an infection, my body will run a fever and try and get rid of the foreign visitors however it can. Tolerance, on the other hand, means the body does damage control instead of killing the pathogens. More tolerant animals won't face severe symptoms, even if they have lots of bad stuff in their body, because their body has figured out a way to keep the cells and tissues from being damaged.