Better to say, the viri killed off more of those without the gene, but those with it lived to produce offspring they passed this to. I don't think that people normally acquire a mutation except at conception.
"I don't think that people normally acquire a mutation except at conception."
The hypothesis is, I believe, that a virus can insert itself into the cells that produce the gametes and transfer the mutation to offspring. So you are correct, but this is a new mechanism they are investigating in the study of the mechanisms of evolution.
You understand that a mutation doesn't necessarily cause an immediate change in phenotype. The mutation could be in the junk DNA and not activated.