Mosquitos are nothing new to the region. This large specimen was spotted in 2010. (Photo: Chris Devers)

Four Cambridge mosquito samples have tested positive for West Nile virus, along with several from nearby communities, leading the state Department of Public Health to raise the risk level to “moderate” from “low” in Cambridge and the surrounding area.

“Most human West Nile virus infections are mild, but a small number of people become very sick,” said Claude Jacob, the city’s Chief Public Health Officer and director of its Public Health Department, identifying people over 50 as being at the greatest risk for serious illness, especially those with weakened immune systems. Symptoms tend to include fever and flu-like illness.

West Nile virus is usually transmitted to people through the bite of an infected mosquito.

“We are asking residents to remain vigilant and take added precautions this season by avoiding mosquito bites and removing stagnant water near their homes,” Jacob said.

So far this summer, the East Middlesex Mosquito Control Project has treated more than 5,500 storm drains in Cambridge with larvicide, which kills mosquito larvae before they can grow to adulthood, health officials said.

The communities raised to a “moderate” risk level are Cambridge, Somerville, Watertown, Boston, Brookline, Chelsea, Everett, Malden, Medford, Melrose, Newton and Revere.

Although not all of the communities identified have had mosquito positives yet, state health officials said, “historic evidence shows that WNV activity tends to follow similar patterns throughout all these communities.”

No human cases of the virus or Eastern Equine Encephalitis have been detected so far this year, but last year, there were six human cases of West Nile infection identified in Massachusetts.

State Public Health Veterinarian Dr. Catherine Brown said the recent findings “serve as an important reminder of the need for personal protection from the threat of mosquito-borne illness,” and offered these tips:

“Protection includes using a mosquito repellent with an EPA-approved ingredient according to the directions on the label, using clothing to reduce exposed skin when weather permits, draining standing water to prevent mosquito breeding and repairing window screens to keep mosquitoes out of your home.”