West Nile virus infections in South Carolina are climbing as officials warned Wednesday that people should take steps to protect themselves from the mosquito-borne disease.

“We felt it necessary for folks to understand that the threat of West Nile virus is still out there,” said Jim Beasley, spokesman for the state Department of Health and Environmental Control.

The number of people who have been infected with the virus now stands at 41 statewide, including two cases in Charleston County and one in Berkeley County. That is up from 28 cases reported in South Carolina about three weeks ago. Three deaths caused by West Nile happened in August in Aiken, Darlington and Cherokee counties. Until this year, only one person in South Carolina had died from West Nile in more than a decade of tracking the disease.

“Mosquitoes can be active in the fall, even after extended periods of cold weather,” said Chris Evans, an entomologist with the DHEC Bureau of Laboratories. “Some mosquitoes, like the one that transmits West Nile virus, overwinter as adults. They seek shelter on cold days, but will come out on warm days to feed.”

Insect repellent containing DEET and protective clothing are recommended, particularly in the early morning and late afternoon when exposure to mosquitoes is most common, DHEC said. Doors and windows should have tight-fitting screens. Eliminate standing water in places such as tire ruts, flowerpots, old car tires, rain gutters and pet bowls.

October and November can be a big time for mosquitoes, said Robert Weinschenk, owner of Knockout Mosquito System.

“You have to get a freeze,” he said.

Of the 41 reported West Nile cases, 20 were more severe forms of the disease that affect the nervous system and cause inflammation of the brain, the spinal cord or the membrane around the brain and spinal cord. Nine infections did not invade the nervous system but caused symptoms such as West Nile fever. Twelve people had no symptoms of infection, but their donated blood tested positive for the presence of the virus, the Centers for Disease Control and Prevention said.

In addition to human infections, the West Nile virus has been detected this year in 24 birds, five horses and nine mosquito samples. Another mosquito-borne illness, eastern equine encephalitis, has been found in one person and 15 horses, Evans said.

This year, more than 5,000 cases of West Nile, including 228 deaths, have been reported in the U.S., which is the highest number of cases through the first week of November since 2003, the Centers for Disease Control and Prevention said.

More than 30,000 people in the U.S. have been diagnosed with West Nile since 1999, and of those almost 13,000 have been seriously ill and more than 1,200 have died, the CDC said.

Some 80 percent of people infected with West Nile don’t get sick or have any symptoms. About one in every 150 people infected will become seriously ill. Milder symptoms lasting several days, such as fever, headache and body aches, happen in about 20 percent of people infected with the virus, DHEC said. The incubation period for West Nile is thought to range from three to 14 days after a person is bitten by an infected mosquito.

West Nile was first detected in South Carolina in 2002 and first found in the U.S. in New York City in 1999.