Four poultry workers in Colorado who recently contracted bird flu bring the total of human cases in the U.S. to at least nine.
Though that number is small, disease researchers say commonalities between the cases — all but one of which were reported in the last four months — are enough to start assembling a picture of how the virus may affect people.
The U.S. cases have been relatively mild and limited to farm workers who had handled infected animals — a sign that the virus in its current state isn’t a major threat to humans.