Only in very severe disease would that be the case. Many can become maimed & debilitated instead of just dying(e.g. Polio), and even those who come through "unscathed", the damage may just be hidden. You can see this with a lot of diseases like HPV, herpes(chickenpox/shingles) or hepatitis, where severe disease manifests well into adulthood. Many diseases have adapted to not kill the host and evade the immune system.
You also may just have chronic re-exposure or a failure to clear pathogens, which can be detrimental to an entire population. An example would be worms that don't get cleared by the immune system and for which the person is constantly being re-exposed to. You end up with cognitive and physical issues that don't kill, but reduce a person's quality of life.
Some of the hygiene hypothesis is around the immune system amping up its response due to parasites, and without the parasites present to attack it turns against the body itself. This is kind of what I am talking about. The people with amped up immune systems possibly handled parasite infections better when they were a common problem, but in a society where most people no longer live with chronic parasite infections, it manifests as allergies/immune disease. Living with chronic parasite infection is certainly not on the top of anyone's list to do, though helminthic therapy is something some people have tried to fix their allergy issues.
It is also incredibly hard to determine who has truly lived a "pure" lifestyle free from interference of modern conveniences, so that evolution can truly be the driving factor in their life. Civilization has disrupted evolution for millennia by now, we can't just look back a few generations and claim they were a "tougher breed" who were better because of their purer natural lifestyle.
You also may just have chronic re-exposure or a failure to clear pathogens, which can be detrimental to an entire population. An example would be worms that don't get cleared by the immune system and for which the person is constantly being re-exposed to. You end up with cognitive and physical issues that don't kill, but reduce a person's quality of life.
Some of the hygiene hypothesis is around the immune system amping up its response due to parasites, and without the parasites present to attack it turns against the body itself. This is kind of what I am talking about. The people with amped up immune systems possibly handled parasite infections better when they were a common problem, but in a society where most people no longer live with chronic parasite infections, it manifests as allergies/immune disease. Living with chronic parasite infection is certainly not on the top of anyone's list to do, though helminthic therapy is something some people have tried to fix their allergy issues.
It is also incredibly hard to determine who has truly lived a "pure" lifestyle free from interference of modern conveniences, so that evolution can truly be the driving factor in their life. Civilization has disrupted evolution for millennia by now, we can't just look back a few generations and claim they were a "tougher breed" who were better because of their purer natural lifestyle.