Baltimore, Anne Arundel and Prince George's counties are getting sprayed for mosquitoes and health officials are warning residents to take precautions as the nation copes with the worst season of West Nile Virus since the disease was discovered in the United States in 1999.

One person in Maryland has died from the disease this summer and 21 have contracted it, according to the Maryland Department of Health and Mental Hygiene.

Nationally, the potentially fatal disease, spread by mosquitoes that pick it up from infected birds, has afflicted 1,993 people, according to the Centers for Disease Control. Nearly every state has been affected. The number of cases leapt 25 percent last week and there's still roughly a month to go in mosquito season.

Neighboring Washington, D.C., and Delaware also reported one death each from West Nile last week. In Delaware a 76-year-old woman with several underlying medical conditions died from the disease. Maryland wouldn't release any details about its West Nile-related death.

State officials say there is no need for panic. Maryland cases so far are only slightly higher than a typical year. Last year, 20 people contracted West Nile and one person died in the state. But they want people to be aware of symptoms of West Nile and are spraying for mosquitoes in areas where the disease has been detected.

Other parts of the country have been hit hard by the virus. The CDC said that most of the country's outbreaks are concentrated in a few areas, including Texas, which accounted for nearly half of the reported cases and has had 35 people die from the disease. Nearly 75 percent of cases were reported from six states.

"Lucky for us, we're not having the same increase in West Nile activity as in other places," said Michael Cantwell, program manager of the Maryland Department of Agriculture's mosquito control section. "Our numbers are nothing like they are in Texas."

The department, which manages the spraying program, sprays regularly in 2,100 communities in 23 counties throughout the state, but ordered the special sprayings in the three counties — starting in Anne Arundel three weeks ago — after mosquito pools tested positive for West Nile.

There are no plans to spray in Baltimore City or Howard County.

Cantwell said the state is not beefing up mosquito control because of the nationwide increase in West Nile. Instead, it continues to work with the state health department to randomly collect mosquitoes and test them for disease. They also work with local governments and neighborhoods to control the mosquito population, including treating wetlands to prevent mosquito larvae from developing.

"We're always watching," Cantwell said. "Problems will show up if we do what we normally do."

West Nile virus first appeared in Maryland in 1999 when an infected crow was found in Baltimore City. The first human West Nile cases were reported in 2001 and reached a peak of 73 in 2003.

It is not known what has caused this year's increase of the virus but it is likely due to weather patterns, including a mild spring and heavy rains that caused an earlier start to mosquito season.

"It's hard to pinpoint any one thing because it is such a complex system," said Katherine Feldman, the state public health veterinarian in charge of the mosquito surveillance program. "So many factors in nature vary from year to year."

In Maryland, the mosquito population actually fell in early summer because the climate was so hot and dry. The insects need water to breed. Mosquitoes lay their eggs on water surfaces or near shorelines where water will cover the eggs, Cantwell said. Mosquito larvae need water to fully develop. Maryland's mosquito population has increased in August with more rainfall.

While West Nile may attract a lot of attention, it typically isn't a widespread disease. About 80 percent of people who contract the disease don't show symptoms and never know they have it. More than 19 percent show mild symptoms such as a fever. Less than 1 percent develop dangerous symptoms, such as meningitis and swelling of the brain.

The disease is considered a public health issue because severe cases can lead to death.

Of this year's cases, 1,069 or 54 percent were classified as neuroinvasive disease with more dangerous symptoms and the remainder were classified as non-neuroinvasive disease.

The very old and very young, and those with compromised immune systems, are the most likely to develop dangerous forms of West Nile, but health officials said everyone should protect themselves from mosquitoes as a precaution.