Prior to the advent of modern medicine people didn't heal themselves. They died at record rates from things that are minor inconveniences now.
The advances of modern medicine shouldn't blind us to the fact that humans and other animals/organisms have always had immune systems that fight off disease naturally, and that practices like good nutrition, sleep and physical fitness, and yes, some folk remedies, all assist the immune system to function optimally - even if by placebo effect.
Sure, modern medicine can take over where the body's natural defenses and traditional health practices are inadequate.
But bacteria and viruses have existed for longer than humans have, and as a species we've survived a pretty long time without modern medicine.