I knew about PMR because my father had it around 30 years ago. He later got the more serious form, GCA, and almost died from it after a month in the hospital with no diagnosis. I think perhaps doctors are getting better at diagnosing PMR but it has been around. The encouraging part of the story is that my father lived to 103 with very good mental and physical function up until a few weeks before he died.
I notice that many people posting on this forum write that they have had healthy lifestyles. Of the 6 people in my birth family I would say that my father and I fit that profile more than other members: never overweight, exercised, ate a good diet. Maybe we avoided heart disease, cancer and diabetes so something else got us!
Like you I am on prednisone plus methotrexate and am concerned about getting infections from too much immune suppression. When I first got on the methotrexate I had symptoms of a urinary infection and the dose was lowered. I'm in discussion with my rheumatologist now about maybe lowering the dose again due to possible oral thrush. Hopefully these more minor infections will show up before anything major! I recommend staying vigilant and in communication with your doctor.