I’m interested in why some people have immunity to the disease and live to pass on their genes.
People can be infected, survive the disease, and then pass on their genes. However, that doesn’t pass immunity, any more than a vaccinated parent passes immunity to measles to his or her child.
A survivor might have a genetic advantage that enabled his survival. On the other hand, disease outbreaks were episodic. A person might reach adulthood without exposure, which means that he reproduced without reference to any disease-resistant factors.
Many of these survivors had exposure to less than lethal variations of the infectious disease. They developed antibodies but I don’t know if those can be passed on genetically. What could be passed on are vigorous immune systems.
The age group who died most frequently during the influenza pandemic of 1918-19 was 20-40. The theory is that the virus had infected older people in previous years so they had some immunity after it mutated into a virulent strain. I don’t know if there is any explanation for the survival of many children. There were villages in Alaska where all of the adults died leaving only the children behind. The village of Eklutna outside of Anchorage started out as an orphanage for these kids in the early 1920’s. There was a similar orphanage outside of Nome.
Black Death Mutant Gene Resists AIDS, Say Scientists (Virus)
http://www.freerepublic.com/focus/news/1314116/posts?page=45#45
If you read descriptions of the plague from the 1500s and 1600s....there were people who would go through the start-up symptoms, and survive. A lot of these folks ended up as caretakers or body-handlers for disposing of the dead. Oddly, as the plague would come around about every twenty years....these people were pretty much ‘insulated’ and didn’t have nothing to worry about.