District officials said Thursday that the person who died from West Nile virus was an elderly man. D.C. health officials confirmed the fatality Wednesday but provided no details.

The number of West Nile virus cases reported in the United States this year through early September is the highest year-to-date total since the mosquito-borne disease was first detected in this country in 1999, federal officials said this week. The number of fatalities had increased by nearly a third from the previous week, they said.

The mid-Atlantic region has not been heavily affected, although local officials are monitoring the situation. The number of cases reported in the Washington area this year is about the same or fewer than in other years.

In Maryland, 21 cases and one adult death have been reported this year, as of Thursday. Virginia has five confirmed cases and no fatalities. D.C. health officials have confirmed at least one other case, but there is often a lag between the time someone gets sick and when the case is reported to health authorities.

Federal and state health officials say the mild winter, heat waves and rain patterns may be factors in this year’s outbreak, which is hitting Texas and several other states particularly hard.

The virus lives in the blood of birds, and mosquitoes spread it to people and horses.

There is no vaccine for humans and no specific treatment for West Nile virus infection. Seniors and children are the most vulnerable, but the virus is life-threatening and has the potential to affect all people.

West Nile disease can vary in severity. The onset of symptoms can take from a few days to two weeks. People age 50 and older have the highest risk of severe illness.

About 80 percent of people who are infected will not develop any illness. About 20 percent will develop West Nile fever. Symptoms include fever, headache, tiredness and body aches. Occasionally, patients will develop a rash and swollen lymph glands.

The most severe type of infection causes inflammation of the brain or of the tissues surrounding the brain and spinal cord. In those cases, symptoms include headache, fever, stiff neck, muscle weakness and paralysis.

Health officials say people should use insect repellent when outdoors, especially at dusk and dawn, when mosquitoes are most active. They also should eliminate mosquito-breeding areas by emptying birdbaths, flowerpots, buckets and barrels where rainwater collects.