Flu's deadly numbers game

I don't pretend to understand viruses, but I fear them. Here's an edited quote:
'The big concern now is fast-track mutation: that a person carrying a human flu virus also catches avian flu. If that happens, the viruses can trade genes leading, in the worst case, to a lethal and highly infectious virus, which no one has immunity to. . . Mixing avian and human flu viruses - a process called "antigenic shift" - to produce a particularly nasty version can happen without both viruses getting into humans. Pigs can catch flu from humans and birds, making them effective mixing pots. . . One difficulty is understanding what genetic changes could make avian flu virus a global killer. The virus could need genes to make it replicate more rapidly, so you cough or sneeze more virus out. It could need genes that make you sneeze harder or more often. "It's really the $1bn question," says Jacqueline Katz of the US Centres for Disease Control (CDC) in Atlanta. "We really don't understand what would make it transmissible between humans." '