I live a fairly healthy lifestyle. I am pretty active, eat well (well = mostly healthy stuff) but I still get sick (flu-type-o-ill) regularly, more than I should. I adhere to what is generally considered the basics of staying healthy:

• Don’t smoke.
• Eat a diet high in fruits, vegetables, and whole grains, and low in saturated fat.
• Exercise regularly.
• Maintain a healthy weight.
• Control your blood pressure.
• If you drink alcohol, drink only in moderation.
• Get adequate sleep.
• Take steps to avoid infection, such as washing your hands frequently and cooking meats thoroughly.
• Get regular medical screening tests for people in your age group and risk category.

If I had to put a number on it, I would say every second month or so. My wife and close friends who I spend most of my time with, live a similar lifestyle to me and they almost never get sick.

My failing seems to be my sleeping habits. I have gotten better from what I used was ten years back; 4 hours average a night to my current of about 6 hours average (although @eldadviljoen will disagree with me). So this is a tiny health experiment. I will see if I can increase my sleep time to 8 hours average and in a very rough way monitor if I get sick as often still.