AKRON (AP) - Health officials are puzzled over why the West Nile virus, which killed 31 Ohioans just three years ago, has been drastically reduced as a summer threat.

Some say changes in the weather and wider immunity might be the reason the number of cases in humans has dropped more than 97 percent during the last two summers, but they have no definitive answer.

''We like it,'' said Dr. Marguerite Erme of the Akron Health Department about the drop in reported cases. ''But as to why it's occurred - we're not sure.''

More than 400 cases were reported in Ohio in 2002. That number dropped to 108 in 2003, and then to just 12 last year. Eight people died in 2003, and two in 2004.

So far this year, one case has been reported in the state, in a 26-year-old man from Darke County in western Ohio.

The virus is primarily a disease of birds and can be spread to people by infected mosquitoes. It is not contagious from person to person.

Most people infected with the virus show no symptoms at all, or only very mild ones. Symptoms include fever, headache, rash, general muscle aches and weakness, gastrointestinal distress and lymph node inflammation.

Nationwide, the number of cases dropped for the first time last year, plunging from 9,862 in 2003 to 2,539. Deaths fell from 264 to 100.