The issue with that flu strain was that it killed people by an overreaction of the immune system. So, ironically, the people with the strongest immune system—young adults—were the ones who were most likely the die. The elderly and young, with weaker immune systems, were more likely to survive.
If it’s really modeled on the 1918 flu, Herschel, who is elderly, may well get sick, but he’d likely recover. Glen, though, is in some trouble.
Glenn and Sasha are both in some trouble. And she was just starting to get some lines this season!