First human West Nile infection in Maryland

West Nile is transmitted by mosquitoes, in particular a species called the culex.

BALTIMORE, Maryland (CNN) -- Maryland health officials confirmed the state's first human case of West Nile virus on Thursday.

Tests confirmed that a 72-year-old west Baltimore man's encephalitis -- a potentially fatal swelling of the brain -- was caused by West Nile virus, said City Health Commissioner Dr. Peter Beilenson.

The man, who Beilenson said had a history of chronic health problems, was discovered unconscious in his home about two weeks ago. He is currently in a light coma on a ventilator at Sinai Hospital.

Thursday's diagnosis is just the latest in a string of humans infected with West Nile. All of the cases have been outside New Jersey and New York, the only two states where the virus was detected the last two years.

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Health officials in Georgia confirmed last month that a 71-year-old woman died August 17 of the virus, the state's first confirmed fatality due to West Nile.

There have been four confirmed West Nile cases in humans since early July in Florida. But no one has died from the virus, according to the Florida Department of Health.

The virus has also been found in birds in several states, including Wisconsin, New York and Michigan.

Mosquitoes transmit the virus to people, though not everyone bitten by an infected mosquito will become ill. Most people suffer no symptoms or experience only mild ones, with young children and the elderly being the most susceptible.

Those who do have symptoms usually experience them five to 15 days after being bitten by a mosquito carrying the virus. Symptoms can range from mild illness to illnesses characterized by fever, headache, muscle aches, swollen lymph nodes and sometimes a rash. More severe illnesses are rare but may include confusion or disorientation, indicating brain swelling, severe muscle weakness or paralysis.

The first case of West Nile virus in the Western Hemisphere was reported in the New York area in 1999. From 1999 to 2000, the disease infected 82 people and caused nine deaths in New York and New Jersey.