An unwelcome virus.

Byline: The Register-Guard

It was only a matter of time: Oregon could not have gone on forever as one of only two of the 48 contiguous states without a reported case of West Nile virus. Indeed, it's possible that the Malheur County case reported Thursday was not the first; four out of five people who test positive for the disease show no signs of illness and don't even know they have the virus.

But now Oregon will have to join every other state but Washington, and much of the rest of the world, in learning how to minimize a new health risk.

Birds are the reservoir; 138 bird species in the United States have been found to carry the West Nile virus. The virus often kills crows and jays - a dead crow found last week near Sheldon High School has tested positive - but most birds survive, spreading the virus as they migrate.

The disease spreads to humans and other mammals via mosquitoes; people don't get sick from touching birds (though it's sensible to take precautions when handling dead or diseased animals of any kind). The best way to avoid the disease is to avoid mosquito bites.

In urban areas, homeowners can perform a public health service by checking their property for standing water that offers a home to mosquito larvae. Among mammals, horses are especially susceptible to West Nile virus and need to be protected from infection.

The West Nile virus was first identified in the West Nile region of Uganda in 1937. Outbreaks in Egypt were reported in the 1950s, and in Israel in the 1960s. The disease first spread to North America in 1999, and it may already have peaked in the United States.

So far this year, just fewer than 1,200 cases have been reported nationwide, resulting in 30 fatalities. There were nearly 10,000 cases last year, and 264 people died.

The complex ecology of the disease, relying on an avian reservoir and insect transmission, may provide natural protection against an epidemic.

But even if West Nile virus is past its peak nationwide, it's new to Oregon and can be expected to become more common before it begins to recede. That means Oregonians will have to gain an understanding of the disease - recognizing, for instance, that it cannot spread from one person to another - while also keeping the risk in perspective.

West Nile virus is an unwelcome arrival, but Oregon was fortunate to avoid the virus long enough for the rest of the country to provide lessons in coping with it. With a few sensible precautions, Oregon not only can be among the last states affected, but among the least.