1. What is West Nile virus and what happens if you become infected?

Relman: West Nile virus is a member of the flavivirus family, a group of RNA viruses that includes the agents of yellow fever and dengue fever. Most humans with West Nile are unaware of it and don't even get signs or symptoms of disease, but about 20 percent develop fever. Only about 1 in 150 people develop severe disease involving the central nervous system, such as encephalitis. The elderly are at particular risk for more serious complications.

2. How is West Nile virus spread?

Relman: The reservoirs for West Nile virus are various species of birds. Humans are accidental hosts. Nearly all transmission between birds and from birds to humans is from mosquito bites. It has also been passed between humans via blood transfusion but that's much rarer. Stanford Blood Center began screening for the virus last summer, however.

3. Where did the virus come from?

Relman: West Nile virus was well established in the Eastern Hemisphere for more than half a century prior to its arrival in the Western Hemisphere in 1999, especially in Africa and the Middle East. The New York 1999 virus was very similar to a strain that circulated in Israel in the late 1990s. It's not clear whether an infected mosquito, bird or human brought the virus to the United States.

4. We've heard in the news for the last few years about other states' problems with the virus, but it hasn't affected us directly. How worried should we be in Northern California?

Relman: Attempting to predict the behavior of an emerging disease agent soon after arrival in a new ecosystem is hazardous. But all indications suggest conditions in California are ripe for significant transmission of West Nile virus later this year and establishment of the virus in many regions of the state.

5. How can we protect ourselves?

Relman: Avoid mosquitoes and mosquito bites. That's the best place to start. Mosquitoes are most active at dawn and dusk. Wearing long-sleeved shirts and long pants, as well as using repellent can help. Also get rid of standing bodies of water that can support mosquito breeding when possible. If you find dead crows, ravens, magpies, jays, sparrows, finches and raptors report them to local health officials for testing. There are no vaccines or drugs for people with West Nile virus disease but efforts to develop them are under way.