Last year's West Nile virus season was the mildest in eight years, and just one case of serious illness has been reported so far this year.
U.S. health officials on Thursday said there were 386 cases of severe West Nile illness and 33 deaths last year. That's a far cry from the peak years of 2002 and 2003, when illnesses numbered nearly 3,000 and deaths surpassed 260.
West Nile was first reported in the United States in 1999. It's spread by mosquitoes that often pick up the virus from birds they bite. Most cases occur in July through September.
Severe symptoms including neck stiffness, disorientation, coma and paralysis.
One possible reason for fewer cases is that birds may be developing immunity to the virus.