Too much caffiene
You do not have a natural immunity to - let's pick a well understood one - Polio. Any statement about "a strong immune system" badly misses the definition of immunity. You may survive it, which may provide a natural immunity for an indeterminate amount of time (it varies by disease). You may also die.
"I'm a tough guy" is a poor refutation of a well researched, documented, and very publicly tested and proven system.