People are living longer, so we can develop conditions that were undiagnosed a few decades ago. 65 was chosen as the retirement age because something like less than 5% of the working population were expected to reach that age a century ago. People died of so many things when they were young, they couldn't live long enough to develop other problems.
And, as gypsykim said, we can now diagnose problems we didn't even know existed. Did you know cystic fibrosis, as a diagnosis, is only 50 years old? People used to die of it but it was called a dozen different things. The problems I'm having now would have killed me 100 years ago and would probably have been written off as heart failure when heart failure was actually the symptom of an underlying disease that could neither be detected or treated.
People with weak immune systems or genetic problems that would have died in infancy are now living longer and developing problems. If they had died of scarlet fever or smallpox or diphtheria as a child, they wouldn't have lived long enough to develop cancer or whatever.
Yes, we do have toxins in the air, soil and water our ancestors didn't have, which cause problems folks dying of the plague couldn't have imagined but I don't think dozens of unconnected diseases are due to "something" around Miles City. They are different conditions, with different causes, and simply appear because people have to die of something.