KentuckyFC writes "Bird flu gets all the headlines but ordinary flu kills several orders of magnitude more people each year and represents a significant threat to our society. The frightening thing about ordinary flu is how little we understand about how it spreads. According to a report at the physics arXiv blog, researchers trying to model this process say they still don't know some basic probabilities associated with infection (pdf, abstract). For instance, given that the disease has manifested itself clinically in an individual, what are the chances of that person dying? And if a virus can be caught from a number of different host species (as it might eventually be with bird flu) what is the probability of transmission?"