I think it was in 1999 or 2000, and was a really big seller for a non-fiction book. I’d like to request it from the library, but I don’t know the title (maybe just “flu”?)or author 
Possibly “Flu : The Story Of The Great Influenza Pandemic” by Gina Kolata?
Good book.
I think you’re right, and it did come out in 1999, so that would be the right time period I’m thinking of. (I know the book in question was popular when I was in Americorps*VISTA)