According to Dr. Michael Osterholm, an infectious disease expert from the University of Minnesota, understanding the different kinds of influenza viruses can help explain why bird flu is so potentially deadly.
"There really are, in a sense, three different kinds of influenza viruses," he says. "There is that which naturally lives in wild birds. That virus doesn't really hurt us very often."
The second type of influenza, called seasonal flu, is the one most know. Though it is common, this flu virus kills 36,000 people a year, Dr. Osterholm says.
The third kind of influenza virus is the one that has many people extremely worried. It is a flu virus that has mutated from affecting only birds to one that can infect both birds and humans. "That's when we see a pandemic, or a worldwide epidemic," Dr. Osterholm says. "And that's what we worry about."
Why does a flu virus mutate? "When that virus lives inside a bird, it's uniquely made to live inside the cell of a bird," Dr. Osterholm explains. However, "the influenza virus is one of the sloppiest, most indiscreet, most promiscuous viruses we know. It basically doesn't know how to reproduce itself very well, and it makes mistakes all the time. That's called a mutation. Some of those mutations will actually survive, and when they survive, they actually get closer and closer to allowing it to get inside a human cell. Imagine the chicken virus is a key and the chicken cell is the lock. It gets in easily. The human cell is the lock. The chicken key doesn't work well in it. Over time, the mutations change that key just enough so now it readily gets inside a human cell. That's what we're worried about."