Two more Massachusetts residents have been diagnosed with West Nile virus, bringing to five the number who have fallen ill this year with the mosquito-borne disease, state health officials said Friday.

The constellation of recent cases has prompted the state Department of Public Health to warn residents about a high risk for West Nile infection in several communities.

Those communities include Arlington, Belmont, Boston, Brookline, Cambridge, Chelsea, Everett, Medford, Newton, Revere, Somerville, Watertown, and Winthrop.

“Despite the anticipated cooler temperatures,’’ said Dr. Catherine Brown, state public health veterinarian, “the species of mosquitoes that carry West Nile virus will continue to be quite active for at least the next several weeks.’’

Among the two most recent cases, one was a woman in her 40s, a resident of Norfolk County, who developed a mild form of the disease and was never hospitalized, state officials said.

The other recent case was a Middlesex County manin his 80s who was hospitalized, but has been discharged, according to the state health department.

That man apparently is from Arlington. Health officials in that town, which is in Middlesex County, said in a statement Friday that one resident had tested positive for the virus. News of that case and evidence of the virus in multiple mosquitoes in surrounding communities led the state health department to warn Arlington residents they face a high risk of being infected.

The latest cases follow three others in the past month, in Suffolk, Norfolk, and Middlesex counties. Boston’s Public Health Commission reported Tuesday that a man in his 70s was hospitalized with the infection.

Brown urged people, especially older residents and those whose immune systems are compromised, to be especially vigilant about avoiding mosquito bites by using repellent, wearing long-sleeved clothes to reduce exposed skin, and reducing outdoor activity between dusk and dawn.

The virus can infect people of all ages, but people older than 50 are at higher risk for severe disease. Most people who are infected have no symptoms. When present, symptoms tend to include fever and flu-like illness. In rare cases, more severe illness can occur.

Last year, there were 10 cases of West Nile infection reported in Massachusetts.

Kay Lazar can be reached at kay.lazar@globe.com