The government expects the bird flu to arrive in the US this year.
The virus can infect people who come in close contact with infected birds.
It has killed nearly 100 people worldwide.
US researchers have found that the H5N1 strain has evolved into two separate strains in humans that may complicate developing a vaccine and preventing a pandemic.
One strain made people sick in Vietnam, Cambodia and Thailand in 2003 and 2004.
A second, a cousin of the first, turned up in Indonesia in 2004.