An outbreak of West Nile virus has killed four people and left 54 others ill in Louisiana. The mosquito-borne virus first appeared in this country three years ago, in New York. It has spread to 33 states and killed more than 20 people. The virus causes no symptoms in many people, but leaves the old, the very young, and people with weakened immune systems with a condition similar to the flu. In severe cases it can cause encephalitis, a potentially fatal swelling of the brain. A dozen more cases were recently reported in Texas and Mississippi, and researchers say there is no telling where the virus will turn up next. West Nile virus is going to become endemic across the country, said Dr. Susan McLellan, an infectious-disease specialist at Tulane University.