An anecdote of my own: I am in my mid 30s and can run a 46 minute 10k and I have had what you describe twice in my life, both times probably caused by the flu.

An anecdote of my own: I am in my mid 30s and can run a 46 minute 10k and I have had what you describe twice in my life, both times probably caused by the flu. One of the times I had to get penicillin for the pneumonia the other time it resolved itself. You should not underestimate the influenza it can be pretty nasty for your lungs too.