Flu Causes Heart Attacks
Heart attack incidence is so increased by flu that CDC tracks flu via heart attack reports.
Among flu-vaccinated elderly, 10% had a heart attack; among the non-vaccinated, 13%. This is hardly conclusive (not double-blind, etc - health conscious elderly both get flu shots and are in better health for other reasons, etc). But the anecdote that CDC believes increased heart attacks are a good reason to suspect a flu outbreak is convincing (if you trust that they do real science).
I’m not sure whether flu is merely challenging (so vulnerable old folks die from heart attack due to momentary stress), or if it actually damages long-term heart health.
I’ve always passed on flu shots, because recovering from influenza hasn’t ever been that difficult for me. But I suppose high fever might damage my brain one day (or, it turns out, my heart).