Chernobyl melted down on April 26, 1986. So, most of the worst radioactive elements are long gone with large numbers of large mammals living in the area with minimal issue. That said, it may have indirectly contributed to a large die-off in combination with something else.

PS: A good example of a disease + enviermental issue is trench warfare. WWI trench warfare created a breeding ground for disease by both compromizing immune systems and packing large numbers of people together. The resulting pandemic is estimated to have caused significantly more death than the actual fighting.