When I was a child I and friends came down with the flu every so often. It was not fun but far from deadly. I wonder if this didn't build up our immunity system.
The Spanish flu that came out in 1918 was more than just "a" flu. Periodically, one of these horrible strains comes out and people's lives are in danger. And flu really should not be taken lightly. If you have some aches and pains and a low-grade fever, that could be "a" flu, but it's not necessarily "the" flu. Not all influenza strains are killers, but some are. That's how my doctor explained it to me.
Check out some of the books on the 1918 flu. They're fascinating. Here is a link to an interesting site at Stanford about the 1918 flu: http://www.stanford.edu/group/virus/uda/