West Nile virus was first isolated in Africa in 1937. By the end of the century, it had spread to Israel, Eurasia, Asia, and Western Europe, areas where the virus reappears every few years, with an average mortality rate of 10 percent.

For many Americans, men and women over 50, and people with weakened immune systems, West Nile virus translates into a new and potentially deadly hazard. It's transmitted to humans, crows, and other birds through the bite of an infected mosquito. There are three separate species that carry the disease: one species feeds during the day, while the other two feed during the evening and at night. Many of the fatalities in New York were senior citizens who contracted the virus in the evening while working in their gardens.

So far, the best defense against the disease is understanding how it's transmitted, and how to avoid contact with the infected mosquitoes that pass the virus to humans. No one knows for sure how West Nile virus made it across the Atlantic. A specialist in mosquito-borne diseases from the United States Army Medical Research Institute for Infectious Diseases (USAMRIID) speculates it may have entered the U.S. last year via an infected mosquito trapped on a transatlantic flight to New York. "When the plane landed," he says, "the mosquito - who hadn't eaten for 7 or 8 hours - headed for the nearest meal, probably a crow."

Precautionary measures you can use

Customs is urging all its employees to educate themselves and their families about West Nile virus. While some states, like Maryland, are setting up virus hotlines (1-866-866-CROW), the best place to get information is still the Internet. The United States Geological Survey (USGS) was one of the first agencies to go online with West Nile virus data - U.S. Department of Agriculture (USDA) and the Department of the Interior both maintain sites that map the spread of the disease. Another primary source of information is a site hosted by the CDC.

Basic facts about West Nile virus

If you don't have time or the resources to do your own research, there are a few facts about West Nile virus you should know. Remember, however, that the information below is just a primer for avoiding what could be a bad encounter with a new illness. Always consult your physician for information specific to your area or relevant to your personal state of health.

• WNV is most dangerous to people over 50, and individuals with weakened immune systems - adults should use mosquito repellent with a high percentage of the ingredient known as DEET (up to 30 percent). Do NOT use repellents containing DEET or potentially dangerous chemicals on children or infants without consulting a physician -- parents should consult a pediatrician regarding appropriate types of mosquito repellent for children of different ages. Always use mosquito repellent when working or playing outdoors, at the beach, and near any body of water.
• Eliminate standing or stagnant water. Check old tires and gutters for standing water and keep animal/pet dishes inside if possible. If it is impossible to eliminate all sources of water outdoors, there are chemicals (Mosquito Dunk, for example) available for use in swimming pools, bird baths, and fish ponds that are often toxic for mosquitoes but harmless to people, birds, and fish.
• Remember that the mosquitoes that carry WNV feed in the morning, at dusk, and at night. Wear cool, light, protective clothing that covers legs, feet, ankles, etc. Do not wear perfume or other substances that may attract mosquitoes. Be careful when working in gardens or around water.
• Do not handle dead birds, if the cause of death is not readily apparent. Advise children not to handle dead birds. Health officials at the CDC say it is unlikely human beings can contract the West Nile virus by handling infected birds but the obvious rule of thumb is "better safe than sorry." If you find a dead crow or other bird whose cause of death is not apparent, call your public heath department. Dogs and cats are not carriers of WNV and are not at high risk, even if they do come in contact with an infected bird.
• Support county and community mosquito control programs. Be responsible for public health education in your community. Initiate dialogue at the local level, share information with at-risk populations, and learn all you can about West Nile virus -- how it is spread and what your community can do to combat it. Funding for spraying programs must be obtained at local or county levels.
• Symptoms of West Nile virus include flu-like symptoms, an abrupt onset of moderate to high fever and sometimes chills, headache (often frontal), sore throat, backache, myalgia, fatigue, conjunctivitis, rash spreading from trunk to extremities and head, nausea, abdominal pain, diarrhea, and respiratory symptoms.

If you are bitten by a mosquito, don't panic. The majority of mosquitoes are not WNV carriers.

If you think you might have contracted West Nile virus, don't panic. The majority of people infected with the disease recover completely. Call your physician or public health department and arrange for the appropriate testing.

If you have information about dead birds, especially crows, that might support the government's efforts at surveillance mapping, or other information about the possible spread of the disease, contact the U.S. Geological Survey or the CDC and ask that your information be added to their weekly updates. Once WNV has been identified in a certain area, scientists note that the disease is likely to reemerge over the course of many years - be aware and make mosquito control a part of your regular routine every year during the summer and fall.

Sources: Research conducted by the Office of Public Affairs through the CDC and the USAMRIID.