The immune system is one of God's wonders. All one has to do is have common sense, eat good, live a natural lifestyle outdoors, take some vitamins, stay away from sick people (unless one are taking care of them), LIVE the faith, and let God sort out the rest.
The problem is that man today has no common sense, eats processed junk foods, live un-natural lifestyles indoors, take opiates ( alcohol, drugs, cigarettes...), go work work and "party" while sick, live as if God did not exist and everything happens by chance.