NEW ORLEANS (Reuters) - The West Nile virus has killed a fifth person in Louisiana and roughly a dozen more have contracted the disease, making the state's outbreak the largest ever in the United States, state health officials said on Tuesday.

Officials said a 76-year-old woman was the fifth fatality from the disease, which has infected 71 people in the state. Since there are three more months left in swampy Louisiana's hot season, officials said they expected more infections.

"There's no doubt that we are going to continue to see a substantial increase in the number of cases. We think the numbers are going to be in the hundreds," said David Hood, the secretary of Louisiana's Department of Health and Hospitals.

The largest previous U.S. outbreak was in 1999 in New York state, where 62 people were infected with the disease and seven died. With the latest death at least 23 have died from West Nile since it arrived in the United States.

West Nile, common in Africa and Asia for decades but not seen in the Americas until 1999, is spread to humans by the bite of an infected mosquito.

Most people who contract West Nile virus suffer nothing more than headaches and flu-like symptoms, but the elderly, chronically ill and others with weak immune systems can develop fatal encephalitis or meningitis, or inflammation of the brain and spinal cord. That occurs in about one of 200 cases, officials said.

Four of Louisiana's fatalities were people older than 70 and at least three had other health problems, making them more vulnerable to the disease, officials said.

At least 34 states, from Massachusetts to Texas and the District of Columbia reported some West Nile activity in 2002, according to the Centers for Disease Control and Prevention. The majority of those cases involved animals.

The outbreak appeared centered around Louisiana, officials said. Texas had eight confirmed cases and two more suspected, while Mississippi had 22 confirmed. All three states have large mosquito populations, which thrive and breed year-round in the warm climate.

Arkansas reported its first human infection on Monday, while Minnesota, West Virginia and South Dakota this week reported cases in birds or mosquitoes for the first time.

In Washington state, health officials said a man visiting there was infected with the disease and determined he must have been exposed when he was in Louisiana. The man had developed only mild symptoms, officials said.

Louisiana asked the Federal Emergency Management Agency for $13.7 million for mosquito-control efforts. Last week Gov. Mike Foster declared a state of emergency in a bid to get easier access to that federal money.

Health officials urged people to be vigilant in protecting themselves. "You cannot count on mosquito control to do it all for you. You have to do your own prevention," said Dr. Raoult Ratard, an epidemiologist with the Louisiana Department of Health and Hospitals.

Most people can avoid becoming infected by wearing long-sleeved clothing, using mosquito repellent and eliminating the standing water in which mosquitoes breed.

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