You've heard the expression whatever doesn't kill us makes us stronger? Well, it some cases, what does kill a few of us, leaves the average of what's left stronger.

Consider a couple real-life examples, starting with MRSA (Methicillin-resistant Staphylococcus aureus). Some people got a staph infection, so the doctor gave them some form of penicillin to fight it. This killed a large fraction of the bacteria in those patient's bodies, and helped those patients fight off the infection. However, in each patient a handful cells out of many thousands survived. And which cells were those? The hardiest ones. Those cells on dividing, spread to new patients, and the process was repeated. Now we have whole strains of super bacteria.

Another example is mosquitoes. Look it up on any outdoors site, and somewhere it will tell you to wear a repellent containing N,N-Diethyl-meta-toluamide, also called DEET. However, outdoor people have been wearing this stuff for decades, and now there is increasing evidence that some mosquitoes are able to ignore it. It is not clear if the same idea was at play, i.e. that sensitive mosquitoes went hungry and died, and the more resistant mosquitoes had a meal, bred, and passed on their resistance. Nevertheless, DEET may be soon be singing its swan song.

For your story, I suggest taking a similar approach. The early exposures do in fact weaken or kill many people, leaving only the strongest and hardiest available to breed. The cycle repeats over a handful of generations, until you have virtually impervious homo superior.

You've heard the expression whatever doesn't kill us makes us stronger? Well, in some cases, what does kill a few of us, leaves the average of what's left of us stronger.

Consider a couple real-life examples, starting with MRSA (Methicillin-resistant Staphylococcus aureus). Some people got a staph infection, so the doctor gave them some form of penicillin to fight it. This killed a large fraction of the bacteria in those patient's bodies, and helped those patients fight off the infection. However, in each patient a handful of cells out of many thousands survived. And which cells were those? The hardiest ones. Those cells went on dividing, spread to new patients, and the process was repeated. Now we have whole strains of super bacteria.

Another example is mosquitoes. Look it up on any outdoors site, and somewhere it will tell you to wear a repellent containing N,N-Diethyl-meta-toluamide, also called DEET. However, outdoor people have been wearing this stuff for decades, and now there is increasing evidence that some mosquitoes are able to ignore it. It is not clear if the same idea was at play, i.e. that sensitive mosquitoes went hungry and died, and the more resistant mosquitoes had a meal, bred, and passed on their resistance. Nevertheless, DEET may be soon be singing its swan song.

For your story, I suggest taking a similar approach. The early exposures do in fact weaken or kill many people, leaving only the strongest and hardiest available to breed. The cycle repeats over a handful of generations, until you have virtually impervious homo superior.