What’s The Oldest Disease?
Having noted that, however, some of the oldest diseases we have direct skeletal evidence for are found on dinosaur species dating to around 250 million years ago, when Pangaea was still a continent and no human ancestors yet roamed the land. “Some of the oldest diseases we have direct skeletal evidence for are found on dinosaur species dating to around 250 million years ago… They include many dinosaurs found with neoplastic tumors, and a dinosaur with tuberculosis-like infectious lesions on the ribs.” Associate Professor, Anthropology, University of South Carolina, who studies health and disease in past human populations using their skeletal remains
The answer to this question depends on what you mean by “disease” and what organism (at risk of disease) you are interested in.
Source: gizmodo.com