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"Encephalitis" means "inflammation of the brain," but it usually refers to brain inflammation caused by a virus. This severe and potentially life-threatening disease is rare.

The illness occurs in two forms — a primary form and a secondary form. The primary form of the disease is more serious, while the secondary form is more common. But because of the milder nature of secondary encephalitis, doctors actually see more cases of primary encephalitis.

Seeing your doctor and receiving timely treatment is important because the course of the disease is unpredictable.

Most people infected with viral encephalitis have only mild symptoms — headache, irritability or lethargy — or no symptoms, and the illness doesn't last long. Serious cases can cause:

• Drowsiness
• Confusion and disorientation
• Seizures
• Sudden fever
• Severe headache
• Nausea and vomiting
• Tremor or convulsions
• Stiff neck — occasionally
• Bulging in the soft spots (fontanels) of the skull in infants

Urgent signs and symptoms may include altered levels of consciousness. In infants, the key signs are a stiff neck and a bulging in the soft spots of the skull. In older children, watch for severe headaches, lethargy, confusion and sensitivity to light. In adults, mental disturbances may be more prominent.

The cause of encephalitis is most often a viral infection. Some examples include herpes viruses; arboviruses transmitted by mosquitoes, ticks and other insects; and rabies and monkeypox viruses transmitted through animal bites.

Encephalitis takes two forms, categorized by the two ways that viruses can infect your brain:

• Primary encephalitis. This occurs when a virus directly invades your brain and spinal cord. It can happen to people at any time of the year (sporadic encephalitis), or it can be part of an outbreak (epidemic encephalitis).
• Secondary (post-infectious) encephalitis. This form occurs when a virus first infects another part of your body and secondarily enters your brain.

Also, bacterial infections, such as Lyme disease, can sometimes lead to encephalitis, as can parasitic infections, such as toxoplasmosis (in people with weakened immune systems) and roundworm infections (transmitted through raccoon feces).

Here are some of the more common causes of encephalitis:

Herpes viruses
Some herpes viruses that cause common infections may also cause encephalitis. These include:

• Herpes simplex virus. There are two types of herpes simplex virus (HSV) infections. HSV type 1 (HSV-1) more commonly causes cold sores or fever blisters around your mouth. HSV type 2 (HSV-2) more commonly causes genital herpes. HSV-1 is the most important cause of fatal sporadic encephalitis in the United States but it's also very rare — about two cases occur per million people per year. Also, you're not more likely to contract encephalitis if you have cold sores.
• Varicella-zoster virus. This virus is responsible for chickenpox and shingles. It can cause encephalitis in adults and children, but tends to be mild.
• Epstein-Barr virus. This herpes virus causes infectious mononucleosis (mono). If encephalitis develops, it's usually mild but can be fatal in a small number of cases.

Childhood infections
In rare instances, secondary encephalitis occurs after common childhood viral infections, including:

• Measles (rubeola)
• Mumps
• Rubella (German measles)

In such cases, encephalitis may be due to hypersensitivity — an overreaction of your immune system to a foreign substance.

Arboviruses
Viruses that are transmitted by mosquitoes and ticks (arboviruses) are the most common cause of epidemic encephalitis. In recent years, these viruses have also produced the most publicized encephalitis cases. Still, this type of encephalitis is fairly uncommon. Here's how the transmission cycle works:

Organisms that transmit disease from one animal host to another are called vectors. Mosquitoes are vectors for the transmission of encephalitis from small creatures — usually birds and rodents — to humans.

Birds that live near bodies of standing water, such as freshwater swamps, are susceptible to infection with an encephalitis virus. When a bird is infected with encephalitis, it carries high levels of the virus in its blood for a short time before recovering from the infection and developing immunity to the disease. If a mosquito feeds on an infected bird, the mosquito becomes a lifelong carrier of the disease. The mosquito transmits the infection to the next bird it feeds on, which in turn passes it to more mosquitoes.

Usually, this transmission pattern cycles through without serious impact on either creature and without affecting humans. This is partly because mosquitoes' primary hosts are birds and small mammals, and they bite humans only as a second choice. But sometimes environmental disasters, unusual weather or other climate changes cause an increase in the number of infected birds, as well as an increased number of mosquitoes. Under these conditions, humans may be affected.

In the United States, the following types of mosquito-borne encephalitis occur:

• Eastern equine encephalitis. As the name suggests, this infection generally afflicts horses. It also can affect humans, although less than 10 cases are reported in most years. Eastern equine encephalitis outbreaks occur most commonly in the eastern United States. Most cases occur in late summer but, in southern states, they can happen year-round. This virus tends to infect birds that live near freshwater swamps. Although some people experience it only as a mild illness, eastern equine encephalitis is fatal in about half of people who develop severe signs and symptoms. Symptoms of eastern equine encephalitis usually appear four to 10 days after a bite by an infected mosquito.
• Western equine encephalitis. Most reports of western equine encephalitis come from the central and western Plains of the United States. The western equine encephalitis virus flourishes in birds that live near irrigated fields and farming areas. Like eastern equine encephalitis, this infection affects horses and, rarely, humans. Human infections are usually first detected in June or July. Symptoms appear between five and 10 days after a bite. Western equine encephalitis is less likely to be fatal than is its eastern cousin, but can result in brain damage and other major complications, particularly in infants.
• St. Louis encephalitis. This virus is transmitted to mosquitoes by birds. The mosquito vector of St. Louis encephalitis breeds in areas of standing water, including polluted pools, roadside ditches and containers such as birdbaths, flowerpots and discarded tires. About 130 cases are reported each year in the United States, although severe outbreaks have affected up to 3,000 people in one year. Symptoms appear within a week to 10 days. Although many young people have mild or no symptoms when infected, the disease can be severe in adults older than 60. The mortality rate is between 2 percent and 20 percent.
• La Crosse encephalitis. This virus is named for La Crosse, Wis., where the virus was first recognized in 1963. It's most common in the hardwood forest areas of the Upper Midwest and in Appalachia. Unlike other forms of viral encephalitis, this virus is passed to mosquitoes from chipmunks and squirrels. La Crosse encephalitis usually affects children and has a mortality rate of less than 1 percent. About 100 cases are reported annually. Symptoms appear five to 15 days after a bite by an infected mosquito.
• West Nile encephalitis. This virus first appeared in the United States in 1999. It's also found in Africa and the Middle East and in parts of Europe, Russia, India and Indonesia. The virus is similar to other encephalitis viruses in that birds are its main animal hosts. However, in rare cases, it's possible for the disease to spread from person to person through organ transplant, blood transfusions or breast-feeding, or from mother to unborn child. Symptoms of West Nile encephalitis are generally mild, but the disease can be severe, especially in older adults and those with weakened immune systems. Symptoms appear within five to 15 days of being bitten by an infected mosquito. In 2003, the Centers for Disease Control and Prevention (CDC) reported more than 9,800 human infections in the United States and more than 250 human deaths. That's more than double the number of infections in 2002, but slightly fewer deaths.

Mosquitoes don't discriminate, so anyone can develop viral encephalitis. But some factors put you at greater risk:

• Age. Some types of encephalitis are more prevalent or more severe in young children or older adults.
• Weakened immune system. If you have an immune deficiency — for example, because of AIDS or HIV — or are going through cancer therapies or organ transplantation, you're more susceptible to encephalitis.
• Geographic regions. Visiting or living in areas of the country where mosquito-borne viruses are common increases your risk of epidemic encephalitis.
• Outdoor activities. If you have an outdoor job or open-air hobby, such as gardening, jogging, golf or bird-watching, be extra careful during an encephalitis outbreak.
• Season of the year. The warm months of summer are the prime mating time for birds and mosquitoes. As a result, mosquito-borne diseases are more prevalent from July through September in many areas of the United States.

You or your child may be severely ill within 24 hours after initial signs and symptoms, or it may take a week before neurological involvement is apparent. With secondary encephalitis, the disease may not develop until seven to 30 days after the onset of the initial viral infection. In any case, it's important to see your doctor as soon as possible if you or your children experience signs and symptoms that suggest encephalitis because the disease can be serious and life-threatening.

Diagnosing encephalitis may involve:

• Spinal tap (lumbar puncture). The most common way to diagnose encephalitis is to analyze the cerebrospinal fluid surrounding your brain and spinal cord. A needle inserted into your lower spine extracts a sample of fluid for laboratory analysis, which may reveal the presence of an infection or an increased white blood count — a signal that your immune system is fighting an infection. Your cerebrospinal fluid may also be slightly bloody if hemorrhages have occurred. Diagnosis of herpes simplex encephalitis can be difficult, but advances in using sensitive DNA methods have allowed detection of the virus in spinal fluid.
• Electroencephalography (EEG). This procedure, which takes about a half-hour, measures the waves of electrical activity produced by your brain. It's often used to diagnose and manage seizure disorders. A number of small electrodes are attached to your scalp with paste or an elastic cap as you recline. You remain still during the test, but at times you may be asked to breathe deeply and steadily for several minutes or to stare at a patterned board. At times, a light may be flashed in your eyes. These actions are meant to stimulate your brain. The electrodes pick up the electrical impulses from your brain and send them to the EEG machine, which records your brain waves on a moving sheet of paper. An abnormal EEG result may suggest encephalitis, but a normal result does not rule out the disease.
• Brain imaging. A computerized tomography (CT) or magnetic resonance imaging (MRI) scan may reveal swelling of your brain. Or it may reveal another condition with signs and symptoms that are similar to encephalitis, such as a concussion. If encephalitis is suspected, brain imaging is often done before a spinal tap to look for evidence of elevated intracranial pressure.
• Brain biopsy. Rarely, if diagnosis of herpes simplex encephalitis isn't possible using DNA methods or by CT or MRI scans, your doctor may take a small sample of your brain tissue. This sample is analyzed in the laboratory to see if the virus is present. Your doctor may try treatment with antiviral medications before suggesting brain biopsy.
• Blood testing. Your doctor can confirm the presence of West Nile virus in your body by drawing a sample of your blood for laboratory analysis. If you have West Nile virus, an analysis of your blood sample may show a rising level of an antibody to the virus, a positive DNA test for the virus or a positive culture of the virus.

Severe viral encephalitis can cause respiratory arrest, coma and death. It may also leave marked mental impairment, which can include loss of memory, the inability to speak coherently, lack of muscle coordination, paralysis, or hearing or vision defects.

Treatment for mild cases mainly consists of rest and a healthy diet, including plenty of liquids, to let your immune system fight the virus. Using acetaminophen (Tylenol, others) can relieve headaches and fever. Anti-inflammatory drugs can help reduce swelling and pressure within your skull. If you're having seizures, your doctor may prescribe an anticonvulsant medication. In some cases, you may also need physical and speech therapy.

Encephalitis can be difficult to treat because the viruses that cause the disease generally don't respond to medications. However, some viruses, particularly the herpes simplex virus and varicella-zoster virus, respond to antiviral drugs such as acyclovir (Zovirax). If you have one of these kinds of virus-induced encephalitis, your doctor will likely start treatment with acyclovir immediately. It's usually administered intravenously while you're in the hospital for at least 10 days. Another antiviral that's sometimes used is ganciclovir (Cytovene).

Scientists are currently investigating interferon therapy — a type of immune cell therapy — as a treatment for encephalitis caused by the St. Louis and West Nile viruses. A pilot study of the treatment showed that patients who received the therapy recovered better than those who didn't receive therapy. However, more studies are needed before the treatment can be approved for these illnesses.

The best way to prevent viral encephalitis is to avoid the viruses that lead to this disease. That means taking steps to prevent genital herpes, for one. It also means making sure you and your children are immunized against chickenpox, measles (rubeola), mumps and rubella (German measles).

To protect yourself and your family against mosquito-borne encephalitis during an outbreak of the disease:

• Dress to protect yourself. Wear long-sleeved shirts and long pants if you're outside between dusk and dawn.
• Apply mosquito repellent. Use 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. 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.
• Avoid mosquitoes. Refrain from unnecessary activity in places where mosquitoes are most prevalent.
• Keep mosquitoes out of your home. Repair holes in screens on doors and windows.
• Get rid of water sources outside your home. Eliminate standing water in your yard, where mosquitoes can lay their eggs. Empty birdbaths, drains, wheelbarrows and flowerpots, and remove old tires and unused containers that might hold water. Drain puddles when possible. Clean your gutters and drain flat roofs regularly.
• Take advantage of nature's mosquito-controlling creatures. Fill ornamental pools with mosquito-eating fish. Consider placing houses for other mosquito eaters, such as purple martins and bats, in your yard.
• Look for outdoor signs of viral disease. Keep an eye out for sick or dying birds and report them to your local health department.

A vaccine is available to protect horses from West Nile virus. No vaccine is available for humans, but several companies are working to develop one.