I'm planning on getting both of the flu vaccines this year - seasonal and swine. Because you know, I hate being sick, and getting inoculated will lower my chances of getting sick. It will also lower my chances of getting other people sick, which is why it's so important for people to (a) get vaccinated even if they are in low risk groups, and (b) STAY HOME IF YOU GET SICK! I don't mean you can't go to the store to buy food, but please, for the love of dogs, don't go to work or to class and give your viruses to everyone you know. Keep them to yourselves.
Whenever I think of the war between viruses and humans, I get to thinking about evolution. Evolution is a fact of the universe, and viruses are an excellent example of how it works.
First, a little virus 101. A virus is a packet of DNA (or RNA) held together in a multi-purpose protein coat that both protects the genetic material and injects it into a host cell. Viruses are not considered living things because they cannot reproduce themselves. They need the machinery of the host cell to replicate both their genetic material and their protein coats.
Even though viruses are not technically living things, they still undergo evolution. Virus use the same kinds of genetic material that we do. (In fact, viruses work because they use the same genetic code that all living things use. If they used a different code, they wouldn't be able to use host cells to replicate.) DNA replication is not perfect. The process produces an error every million or so nucleotides. That isn't a whole lot, but when you're pumping out billions of viruses, you're guaranteed to have some with DNA errors. It's just a fact of how the biochemical processes of DNA replication work. Furthermore, after DNA has been replicated, the copies can be damaged by chemicals and radiation (viruses don't wear sunscreen) which can lead to mutations. So, out of the billions of viruses pumped out by a single infected person, some of them are going to be mutants. Most mutants will be less proliferate than their non-mutant siblings, but some will be as good as or better than their siblings at surviving and proliferating. Since viruses are under constant attack by our immune systems, the ones that are most effective against our immune systems will naturally come to dominate the virus population.
It isn't necessarily about traits that make the virus intrinsically better at survival inside a generic host. If a virus achieves a neutral change to the shape of its protein coat, that can help it to escape recognition by a host that has had that viruses ancestor in the past. You can get basically the same type of flu every year, so long as it has mutated enough that your immune system no-longer recognizes it. And there's nothing you can do to stop viruses from evolving. As long as there are hosts for the viruses to infect, and as long as there are mutagens and imperfect DNA replication, there will continue to be new variations of viruses, and the most prolific variants will spread. Random change + nonrandom reproductive success = evolution.