Having a good diet isn't in any way competitive with being up to date on your vaccines. And a good diet absolutely will do something to prevent most infectious

Having a good diet isn't in any way competitive with being up to date on your vaccines. And a good diet absolutely will do something to prevent most infectious diseases - if your immune system is stressed/compromised due to a shit diet, you're much more likely to be infected when you're exposed. You're exposed to infectious disease constantly, and the reason you don't get sick the vast majority of the time is because you're successfully fighting those off.