Viruses can evolve in a short time frame. Consequently new booster shots are required annually for viruses like influenza. To rapidly evolve, viruses use recombination, which involves swapping of genetic information within specific genes. This is accomplished by a "copy choice" method in cells infected by two distinct viruses.
The recombination mechanism recycles polymorphisms in various combinations. These polymorphisms have already been selected over millions of years, and the virus simply creates new combinations to evade immune surveillance or drug treatments. Recombination follows specific rules that can be used to predict the sequence of emerging viruses.
So why are not all suspected cases of H1N1 NOT being "typed?"
Unsubtypables on the rise; http://www.recombinomics.com/News/02081101/trH3N2_PA_Unsubtypables.html
CDC Silence on unsubtypables deafning;