The Pennsylvania Department of Health said the first probable human cases of the West Nile virus have occurred not far from the Lehigh Valley, in Berks and three other eastern counties.

The presence of the potentially dangerous virus, which is usually transmitted to humans through the bite of an infected mosquito, was reported Friday by the Pennsylvania Department of Health. Other human cases have been detected in Lancaster, Luzerne and Philadelphia counties, the state said in a news release.

Specimens will be forwarded to the Centers for Disease Control and Prevention for confirmation, the state said.

Most people infected with the virus do not feel sick, according to the CDC. About 1 in 5 people who are infected develop a fever and other flu-like symptoms, and about 1 out of 150 infected people develop a serious, sometimes fatal, illness, according to the agency.

State officials recommend that residents minimize their exposure to mosquitoes. Health experts said one way to do that is clean up areas where standing water collects, because that is where mosquitoes tend to breed.

For a fact sheet on the virus, go to the Health Department’s web page, or call 877-PA HEALTH (877-724-3258).

The virus was first detected in North America in 1999 and in Pennsylvania in 2000. The disease maintains itself in nature as mosquitoes bite and infect birds, which infect other mosquitoes that also feed on those birds.