Rhode Island’s first human case of West Nile virus this year was confirmed last week, in a person with a weakened immune system.

The state Department of Health said that the infected person lives and works in Rhode Island. The department would not provide any additional information, including whether the person had been hospitalized.

The last time a Rhode Islander came down with West Nile was in 2006. There were no cases in 2005 or 2007.

West Nile virus is spread by mosquitoes that bite infected birds and then bite mammals. Four out of five people who become infected with West Nile have no symptoms.

Some people suffer symptoms such as fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back, usually for a few days.

Only about one in 150 people develop severe illness, which can involve fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, vision loss, muscle weakness, numbness and paralysis. Those most likely to get sick with West Nile are older than 50. Young children and people with weakened immune systems are also at risk.

The number of infected mosquitoes increases over the course of the summer, so that by late summer and early fall the virus is widespread and people are at greater risk of getting bitten by an infected mosquito.

“We know that West Nile virus is established in Rhode Island’s mosquito population and that is why we stress the importance of personal protection measures,” Health Director David R. Gifford said in a statement. “In particular, people should avoid outside activity at dawn and dusk, use bug spray with DEET, cover strollers and playpens with mosquito netting and repair holes in windows and screens.

“Everyone should continue to take these precautions until the first heavy frost.”

According to the U.S. Centers for Disease Control and Prevention, by the end of September, states had reported 918 cases of West Nile infection, and 19 deaths. There were five cases in Connecticut this year.

ffreyer@projo.com