From Princeton University:
Before wolves were reintroduced to Yellowstone National Park in the mid-1990s, they were vaccinated for common diseases and treated for any parasite infections they already carried. As a result, the first few generations of wolves were relatively disease-free, but over the years, various diseases have found their way into the population.
Sarcoptic mange, caused by the microscopic mite Sarcoptes scabiei, first crawled its way into the reintroduced Yellowstone wolves in 2007. Mange is characterized by itchy skin lesions and hair loss, but there is huge variation between individuals in terms of how sick wolves actually become — some experience mild mange symptoms and are able to clear the mites within a few months, while others experience massive hair loss, emaciation and secondary bacterial infections, and eventually die of the disease.