I worked in primary care and emergency medicine for 46 years. I used to get sick with viruses and other infectious diseases in my early years. Then I stopped using alcohol and other drugs and started to take a variety of supplements every day. I cannot recall the last time I had an upper respiratory infection in spite of being subjected to them almost daily for years. I think one’s immune system needs exercise just like all of the other systems. If you feed it the right types of nutrients, control your stress levels, and get adequate rest you just might be OK.
This has been my experience also. Stopped smoking dope and later cut alcohol way back. Vitamin D and a multi vitamin every other day and havent been sick in years. Also exercise every day. Stress kills I think.