To become immune, you have to catch the disease/virus. Or, have had ancestors, who had it. Only then, will your body develope antibodies, to kill it.
That’s how vaccines work. You are essentially given the disease/virus, in a very small amount. Then, your body makes a defense system, to fight it.
I had, what doctors called, a light case of chicken pox. It sucked, but I have seen far worse cases. Now. I’m immune.
I guess I am saying that the best way to become immune, is to not get the full blown version, and if possible, you can develop a defense for it. Some diseases/viruses, have no cure.
But. Dehydration, and other variables, play a big role in survival. With intravenous fluids, antibiotics, and blood transfusions, we can survive many formerly deadly issues.
Modern medicinal treatments, can keep us alive, until most diseases/viruses, run their course.