All flu viruses - human, bird, pig - have an “H” and an “N” in the name, each followed by a number (the avian flu strain that has been worrying people is H5N1, for example).

　　The letters refer to two proteins (hemagglutinin and neuraminidase) on the surface of the virus. The numbers refer to slight variations in the form of each protein. The variations are important, because our immune system hones in on those proteins to attack the virus.