So the average lion prefers to eat people with weak immune systems? I'm not getting this.
You cannot see how a virus or bacteria can be considered a predator? Or if you really meant to ask about lions, of course they pick off the weak. It's less work. This leaves the herd's average health better after the predation.
Now, suppose we stopped inoculations, and people started dying of these preventable diseases in large numbers. Would this make the species healthier, or just resistant against threats we've already got handled?
Both. Healthier individuals would have a greater chance of survival, and thus a greater chance of passing on their genes. People born with congenital heart failure, asthma or a variety of other conditions would have a higher risk of dying, and less chance of passing on their genes.
There's a by-country correlation between longevity before and after the Spanish Flu. In countries that got hit, longevity increased. Weaker individuals got culled more than healthier ones, and the net result after a generation is a healthier population.
Now, we're seeing the opposite. The number of people with defects (like, but in no way limited to, asthma) is going up. We put great effort into keeping the weak alive and able to reproduce. With a very predictable result: the defects flourish when there's no evolutionary disadvantage to having them.
If you want me to go along with killing large numbers of children that we could save, you're going to have to have something more specific than "increasing the average health of the herd".
How about the overall human health being at a higher level, so when a new marburg/ebola type virus catch us out of the blue, we have a higher chance of survival?
How about when the temperature and humidity raises across the globe, and many of us are too weak to survive it?
Or any number of unforeseen things that may happen, in which a healthier population has less risk of extinction?
Compassion for the weak and exceptionally strong parental instincts might have been a good survival trait in the past, given our long reproductive cycle. But that's no longer a concern. We're not just a few packs on the African plains struggling to survive despite 9 month pregnancies and 12+ years before becoming reproductive. Every life counted back then.
We're now billions of people, and propping up the weak is now detrimental to us as a species. A few tens of thousand deaths a year is now a negligible price to pay for humanity as a whole, to reduce the creep towards the average human being less healthy.