The only time I had the flu was when I was in the Army FORTY years ago. I have always attributed it to:
1. Crowded living conditions. I was in Jump School at Fort Benning and we were packed in like sardines. I read a book about the 88th Infantry Division in WWII which said in WWII the Surgeon General forbade double bunking to reduce the chances of contagion. They knew something that somebody else forgot. Or chose to ignore.
2. Not being able to get off my feet. Yes, I went on Sick Call, the doctor gave me an injection, wrote me a profile. Those of you who have been in the Army know that a profile is not worth the paper it's written on. I am no MD but IMHO NOTHING takes the place of proper rest.
3. Not taking Vitamin C. I have been taking Vitamin C for about 10 years now, I am on my bicycle all year round, outdoors much of the time, it's been so long since I had a cold I can't remember when.