Greg, you're right. In 2009, Alexandre Yersin found Yersinia pestis in tooth pulp of a victim from the 14th century. Maybe nobody today thinks that plague was anything else--I think the speculation I read was from before 2009. Yersinia pestis @ Wikipedia

I did find a New Scientist article that had something about the bacteria, but not about hereditary immunity. There's something about the appearance of comets being linked to disease outbreaks, which is nutty, but no genetic immunity. The New Scientist, 17 November 1979. This may be the wrong article, but it's the closest thing I could find.

Regarding flu shots, if you don't care about potentially infecting other people because you'd rather risk getting the flu than get the shot, then go ahead. Just know that you can transmit it to other people before you feel sick, and there are people who can't get the shot. If you are one of those folks, then don't get the shot. It IS killed virus and can't give you the flu. The symptoms you get occur because of your body's immune response, and shouldn't last very long. It takes a couple weeks to be effective, and if you've already been infected, you'll get sick before then.

In any case, I hope people stay healthy or get that way soon!