That’s a very interesting question – I think you are probably right.
It is certainly true that some infectious disease have been very deadly. I think about 95% of the native American population were wiped out by the smallpox virus that was introduced when Spanish explorers turned up in the wake of Christopher Columbus.
And in the 14th Century about one third of the population of Europe died of the ‘black plague’ (bubonic plague – caused by a bacterial parasite – Yersinium pestis).
But in both those cases, people did survive.
These days it has become apparent that some individuals are resistant to infection by HIV, the virus that causes AIDS.
In each case of course it is the people who happen to have resistance who survive and the resistance is then passed on to their offspring.
It is believed that many human disease have arisen from animal diseases which have switched their ‘preference’ for humans. This probably happened quite frequently when farming with animals started thousands of years ago. One reason why the Spanish killed off so many Americans with their germs is that they were already immune to viruses that the Americans had never been exposed to (the Americans did much less animal farming and had different animals anyway).
For this reason, people take the threat of bird flu very seriously – since this is a disease that might make the jump to humans who have no immunity to new types of flu virus.
If you want to read more about this, a great book on the topic (and much else) is “Guns, Germs and Steel” by Jared Diamond.
I think this is a really good question, and its exactly the sort of thing that gets biologists excited.
Variation in the population is useful, because even if some individuals are vulnerable, others might be protected. So even if it seems that one immune response is ideal at the present, it would be useful if the immune system preserved some variation, or had mechanisms to generate new varied responses to disease.
Looking at it from the virus’s point of view, it would not be great to wipe out the human race as they would lose a host which they need to reproduce. I wonder if viruses have evolved mechanisms which protect them from being too lethal.