Three cases of West Nile virus have been confirmed in the Baltimore area by the Maryland Department of Health and Mental Hygiene, the first appearance of the disease in the state this year.

The virus, spread by infected mosquitoes to humans and other animals, has also been detected in mosquitoes collected at trap sites in Montgomery County and Worcester County, health officials said.

“This is the time of year we see West Nile virus cases in Maryland,” said health secretary Joshua M. Sharfstein. “Our first confirmed cases of the season should serve as a reminder for all Maryland residents to take the basic steps that can reduce the risk of getting infected.”

Authorities recommend limiting outdoor activity around dawn or dusk, when mosquitoes are most active. Wearing insect repellent, as well as long-sleeved shirts, long pants and hats, can also help prevent bites. Residents are asked to patrol their property for standing water that could provide a breeding ground for mosquitoes.

Health officials said that the three people who have been infected are adults.

Most people who contract the virus have no symptoms, the officials said. Those who do generally will experience flu-like symptoms such as fever, headache, body aches, skin rashes and swollen lymph glands.

About 1 percent of West Nile virus cases result in severe infections that require hospitalization, according to health officials. The danger of a serious reaction is greater for people over 50 or with compromised immune systems.

Whenever a case is reported in Maryland, the state Department of Agriculture adds anti-insect sprayings within a mile of the infected person, officials said.

Last year there were 47 cases of West Nile virus in Maryland and almost 6,000 nationwide, the second-highest numbers on record. The Centers for Disease Control and Prevention began its West Nile surveillance program in 2000, a year after the virus was first found in North America.