There are hundreds of types of flu virus out there and new variations form every year. I've read that some of these can actually pass through sexual contact as well. Obviously, kissing someone or being in touch with saliva in any way can pass the virus.
The thing is though, since it's so easy for the virus to spread, it's kind of inevitable in areas where people are in close proximity like cities. Thankfully, the seasonal flu virus isn't very dangerous and most people get over it in a week. But if a more dangerous type of flu virus made entry into the US like Avian flu, then it could spread quickly and make many people very sick.