Six people on Long Island have been infected this year with West Nile virus, a mosquito-borne and potentially fatal illness that has sickened three Nassau residents and three more in Suffolk, health officials in both counties said Monday.

“There is no discernible trend,” said Dr. James Tomarken, Suffolk’s health commissioner. “We know only about the cases in which the patient sought treatment and we received laboratory confirmation of West Nile virus. There may be many more residents who acquired West Nile virus, but we never learned about them because they didn’t seek medical attention or they sought attention but lab tests weren’t ordered.”

Officials said the tally of human West Nile virus victims can vary from year to year and that it typically strikes between June 1 and Nov. 1.

In Suffolk, there were five human cases in all of 2015, one in 2014, four in 2013, 14 in 2012, four in 2011 and 25 in 2010 — when three people died from the disease.

This year, all of the Nassau cases occurred in the Town of Hempstead, and two of those victims were over 50 years old, said Mary Ellen Laurain, spokeswoman for Nassau’s department of health. In Suffolk all three victims were over 50 years old and residents of Islip, Brookhaven and Smithtown, according to Suffolk’s health department.

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Last year, Nassau had reported five cases as of Sept. 26, and four human cases for all of 2014.

Officials said the virus is transmitted to humans through mosquito bites and that 20 percent of people bitten will display symptoms including fever, headache and body aches, skin rash and swollen lymph glands, with more severe symptoms ranging from high fever to convulsions and even paralysis or a coma.

Officials said people most at risk of infection include those over 50 years old and battling chronic illness or stricken with compromised immune systems.

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Officials recommend using insect repellent containing DEET or permethrin, staying indoors from dusk to dawn, and wearing long sleeves and pants when going outdoors during those hours. They also advise eliminating standing puddles of water in flower pots, gutters, birdbaths, swimming pools and hot tub covers.