PORTLAND (AP) -- Four Oregon residents have tested positive for the West Nile virus, the state's first human cases of the year, health officials said Wednesday.

Three of the four had symptoms of the disease and recovered. The fourth person, a Lane County man, had no symptoms, but was found to be positive after giving blood, said Dr. Emilio DeBess, the state public health veterinarian.

DeBess said the patients with symptoms did not have to be hospitalized. They were weak and had fevers ranging from 99 to 101 degrees; one developed a rash.

Their ages ranged from the late 20s to the early 40s.

Besides the man from Lane County, those contracting the virus were a Benton County man and women from Marion and Malheur counties. The residents from Benton and Marion counties are believed to have gotten it from their travels in central California.

"Both reported to have tons of mosquitoes around them," DeBess said.

The nation's blood supply has been screened for West Nile virus since the summer of 2003, and DeBess said the Lane County man's blood did not enter the supply.

"The blood supply is really safe at this point," DeBess said. "They did a great job in picking it up and not passing it on to anyone else."

The cases originated in mid-July, DeBess said. The first positive tests were conducted at the state public health laboratory in Portland, and later confirmed at a lab in Richmond, Calif.

DeBess said the California lab is testing about another 10 Oregon cases. Given the lag time between sickness and confirmation of the virus, DeBess expects more human cases to be detected before the end of mosquito season.

West Nile arrived in Oregon in 2004. It sickened five people, and killed 19 birds and 22 horses.

People become infected from mosquito bites, not through contact with birds, horses or other people exposed to the virus. About 80 percent of those who become infected don't realize it and have no symptoms. About 20 percent of infections result in mild illness.

However, in about 1 percent of those infected, the virus can cause neurological disease, which can be fatal, especially in the elderly.

Nationally, at least 16 people have died from West Nile virus through Aug. 30 of this year, according to the Centers for Disease Control and Prevention. One hundred people died from the virus last year and 264 were killed in 2003.

Precautions against getting West Nile include wearing insect repellent, staying indoors at peak times for mosquitoes, wearing long-sleeved clothing and eliminating standing water where mosquitoes can breed.

"Protect yourself. Protect yourself. Protect yourself," DeBess said.