It's still all theory, but UC Davis researchers think that the reason senior citizens over age 65 aren't catching the H1N1 flu at least for now is that specific shared molecular sites called epitopes result in some type of immunity to the novel flu. It seems people over age 65 have been exposed to various type A flu viruses in the past that have stimulate immunity responses in which their T-cells clear out the novel flu viruses. It's a fascinating theory because some doctors have been heard to