>"normal" plagues will have the problem that reducing the population will lower probability of new infection. Herbert's idea of gender selective mortality is a clever way to jump that hurdle neccessary&imminient in the plot - but for real pandemics species crossing vectors are necessary as travel will be stopped as much as possible. Now if the vector would include some flying birds or cockraches hitching a ride with food transports could hit us like fleas and rats in the middle ages

In that case, start producing and eating food, not the industrial junk sold as "food". That should put you far ahead of the medieval people, who were undernourished and thus didn't have much of a chance with their immune systems. Simple hygiene, and real food.