I would say in the last 70 or so. quinine, penicyllin, sulfa drugs, polio vaccine all let millions without top notch immune systems live. Before then the random combinations which led to weakness were ruthlessly purged from the pool. We benefit from their productivity, but their children pass on potential weakness.
Simple biology. I hope the medicine makers continue their good work. When they can't, we will have a "mass extinction" event. The ones that survive will get to begin again.
The alternative: the genetic manipulation that was intially funded to investigate AIDS-HIV can patch genetic weaknesses such as that which leads to diabetes or autoimmune stuff.
In the past, any such rare autoimmune disease was thought to be just part of life/death. It hid in plain sight as part of the usual mortality rate. Now we keep such people alive longer, and our science can isolate it from other causes of death. No change in the gene pool is necessary, but we sure can extend the lives of people who would have otherwise died. If they breed, their traits are passed on.