INFECTIOUS DISEASE Viral Illnesses • Chickenpox • Mumps • Postherpetic neuralgia • Croup • Measles • Rubella • Mononucleosis • Smallpox • Parvovirus infection • West Nile virus • Roseola • Rabies • SARS • Viral hemorrhagic fevers • Polio • Hand-foot-and-mouth disease

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In the summer of 1999, large numbers of crows began dying in New York City. Health officials eventually discovered the cause, but not before four people died of the same disease — West Nile virus, a mosquito-borne illness that mainly affects birds, humans and horses.

West Nile virus surfaced in Africa in the late 1930s. Since then, it has spread through Asia, Europe, the Middle East, and North and South America. The 1999 East Coast outbreak was West Nile's first appearance in the United States, but the virus is now found throughout the country.

Most people with West Nile virus have no symptoms and never know that they've been infected. Others have only a mild illness and recover fully. But for seniors and people with underlying medical conditions, West Nile virus can be very serious, leading to inflammation of the brain or inflammation and infection of the membranes surrounding the brain and spinal cord. In a small number of cases, West Nile virus can be fatal.

Most people infected with the West Nile virus have no signs or symptoms. About 20 percent of people develop a mild infection called West Nile fever. Common signs and symptoms of West Nile fever include:

• Fever
• Headache
• Muscle aches
• Backache
• Lack of appetite
• Nausea, vomiting and diarrhea
• Skin rash
• Swollen lymph glands

In less than 1 percent of infected people, the virus causes a more serious neurological infection, such as inflammation of the brain (encephalitis) or of the brain and surrounding membranes (meningoencephalitis), infection and inflammation of the membranes surrounding the brain and spinal cord (meningitis), and paralysis. Signs and symptoms of these diseases include:

• High fever
• Severe headache
• Stiff neck
• Disorientation or confusion
• Stupor or coma
• Tremors or muscle jerking
• Signs and symptoms of Parkinson's disease
• Lack of coordination
• Convulsions
• Partial paralysis

Symptoms of West Nile fever usually last a few days, but symptoms of encephalitis or meningitis can linger for weeks, and certain neurological effects, such as paralysis, may be permanent.

In the United States, wild birds, especially crows and jays, are the main reservoir of West Nile virus, but the virus is actually spread by certain species of mosquitoes. Some mosquitoes seem to spread the infection mainly among birds, while others are more likely to transmit it to humans, horses and other mammals.

Here's how the transmission cycle works:

When a mosquito bites a bird infected with the West Nile virus, the virus enters the mosquito's bloodstream and circulates for a few days before settling in the salivary glands. When the infected mosquito bites an animal or a human, the virus then enters the host's bloodstream, where it may cause serious illness.

Although the exact mechanism of illness is unknown, West Nile virus probably enters the host's bloodstream, multiplies and moves on to the brain, crossing the blood-brain barrier — a barrier that separates the blood from the central nervous system. Once the virus crosses that barrier and infects the brain or its linings, an inflammatory response occurs and symptoms arise.

West Nile virus transmission occurs mostly during warm weather, when mosquito populations are active. The incubation period — the period between when you're bitten by an infected mosquito and the appearance of signs and symptoms of the illness — ranges from three to 14 days.

In rare cases, it's possible for West Nile virus to spread through other routes, including:

• Organ transplantation and blood transfusion. Some people have developed West Nile virus after receiving a transplanted organ or blood products from an infected donor. Although donated organs are not yet screened for West Nile virus, blood donor screening for West Nile was instituted in 2003, substantially reducing the risk of infection from blood transfusions.
• Mother to unborn child. In 2002 in New York, a woman in her last trimester of pregnancy contracted West Nile virus. When her baby was born five weeks later, doctors discovered the presence of antibodies to the West Nile virus. The baby had retinal problems and severe damage to portions of the brain. This case was the first evidence that the disease could be transmitted from mother to unborn child (transplacental transmission).
• Breast-feeding. In a few instances, mothers have transmitted West Nile viruses to their infants through their breast milk. But experts at the Centers for Disease Control and Prevention (CDC) say that such cases are extremely rare and should not affect any woman's decision to breast-feed her baby.
• Laboratory acquisition. Some laboratory workers involved in West Nile surveillance and research have contracted the disease from infected animals.

Your overall risk of contracting West Nile virus depends on these factors:

• Time of year. In temperate regions, West Nile virus follows a seasonal pattern that begins in late spring, with the peak time for infection occurring in late summer and early fall — usually August and September. People living in southern climates may run the risk of infection year-round.
• Geographic region. Visiting or living in areas of the country where mosquito-borne viruses are common — especially the East Coast and Midwest — increases your risk of West Nile virus. On the other hand, West Nile has been reported in all 48 contiguous states, including desert regions such as Arizona.
• Occupation. If you work outdoors, you have a greater chance of being bitten by an infected mosquito.

Even if you are infected, your risk of developing a serious West Nile virus-related illness is extremely small — less than 1 percent of people who are bitten become severely ill. And most people who do become sick recover fully. Among those more likely to develop severe or fatal infections are:

• Older adults
• People with immune systems weakened by HIV/AIDS, long-term steroid use, chemotherapy drugs or anti-rejection drugs following transplant surgery
• Pregnant women
• People with certain genetic mutations

See your doctor as soon as possible if you experience signs and symptoms that might suggest West Nile infection — especially if you're pregnant or breast-feeding. Although most people infected with the virus recover fully, the virus can result in serious, life-threatening illness.

Your doctor can confirm the presence of West Nile virus in your body by analyzing a sample of your blood or the fluid surrounding your spinal cord (cerebrospinal fluid). Signs of the disease include:

• A rising level of antibodies to the West Nile virus. Antibodies are immune system proteins that attack foreign substances, such as viruses.
• A positive ribonucleic acid (RNA) test for the West Nile virus.

If your doctor suspects a serious, West Nile virus-related illness such as meningitis or encephalitis, you may have one or more of the following tests:

• Lumbar puncture (spinal tap). The most common way to diagnose meningitis is to analyze the cerebrospinal fluid surrounding your brain and spinal cord. A needle inserted between the lower vertebrae of your spine is used to extract a sample of fluid for laboratory analysis. The fluid sample may show an elevated white cell count — a signal that your immune system is fighting an infection.
• Brain imaging. In some, but not all, cases, a computerized tomography (CT) or magnetic resonance imaging (MRI) scan can reveal brain inflammation and swelling.

Most people recover from West Nile virus without treatment. Even those who develop encephalitis or meningitis may only need supportive therapy with intravenous fluids and pain relievers. Scientists are currently investigating interferon therapy — a type of immune cell therapy — as a treatment for encephalitis caused by West Nile virus. A pilot study of the treatment showed that people who received interferon recovered better than those who weren't given the drug, but more studies are needed.

The CDC has been working with the Environmental Protection Agency as well as other federal, state and local agencies to prevent future outbreaks of West Nile virus. Efforts to detect and contain the virus include:

• Sampling of mosquito and bird populations for West Nile virus
• Increased surveillance of animals and humans for infection
• Eliminating mosquito-breeding areas
• Increased physician awareness and reporting of the virus so that its spread can be tracked
• Conducting public-awareness campaigns to let people know how to reduce their risk of exposure to the virus

Your best bet for preventing West Nile virus and other mosquito-borne illnesses is to avoid exposure to mosquitoes and eliminate mosquito-breeding sites. To help control West Nile virus:

• Eliminate standing water in your yard. Mosquitoes breed and multiply in pools of standing water.
• Unclog roof gutters.
• Empty unused swimming pools.
• Change water in birdbaths at least weekly.
• Remove old tires or any unused containers that might hold water and serve as a breeding ground for mosquitoes.
• Watch for sick or dying birds and report them to your local health department.

To reduce your own exposure to mosquitoes:

• Avoid unnecessary outdoor activity when mosquitoes are most prevalent, such as at dawn, dusk and early evening.
• Wear long-sleeved shirts and long pants when you go into mosquito-infested areas.
• Apply mosquito repellent with a 10 percent to 30 percent concentration of DEET to your skin and clothing. Choose the concentration based on the hours of protection you need — a 10 percent concentration is effective for about two hours, while higher concentrations last longer. Keep in mind that chemical repellants can be toxic, and use only the amount needed for the time you'll be outdoors. Don't use DEET on the hands of young children or on infants under 2 months of age. Instead, cover your infant's stroller or playpen with mosquito netting when outside. According to the CDC, oil of lemon eucalyptus, a more natural product, offers the same protection as DEET when used in similar concentrations. Other natural repellants, such as citronella and oil of geranium, also offer some protection.

A vaccine is available to protect horses from West Nile virus. No vaccine is yet available for humans, but work to develop a human vaccine is under way.