Just a thought. Since it's so difficult to tell pneumonia from the flu and its ilk, I bought one of the little O2 sensors that fits over a finger. I'm using it now to measure my progress at getting off the oxygen, but in the future I'll use it as a check when I have "flu". If my O2 level goes below 90, I'll call the doctor.
A cheap investment - cost about $20 and up on Amazon: