An excellent point about the flu. How many times do we hear friends and coworkers complain about being sick with the flu? Often, its not really the flu since they can still putz around the office, complaining about how sick they are. Just some garden variety upper respiratory illness. True flu will normally knock you down completely for a while. I've experienced a couple bouts with the flu where I was utterly exhuasted and very uncomfortable. Even when you're over it, you're still not 100% for weeks afterwards.

Ironraven pointed out, quite correctly, that the mortality from Spanish Flu was very small. Only two or three percent. But if only two or three percent of the infected died, think about how many millions and millions more were incapacitated while they were sick, and how many people did not work so they could care for the sick. With modern transportation, you could have a significant proportion of the country out sick at the same time.