Still others, like West Nile fever, have moved from one area of the world to another.

Tests are planned to determine whether that recipient had a mild case of West Nile fever.

Mild disease, generally called West Nile fever, has some or all of the following symptoms:

Clinical characteristics and functional outcomes of West Nile fever.

Elsewhere, West Nile fever was more of academic than public health interest because most cases were mild and occurred sporadically.

The risk that a person will develop severe West Nile fever from the bite of an infected mosquito is low.

Mild, flu-like illness is often called West Nile fever.

Yet the first case of West Nile fever in the Western Hemisphere came as a surprise.

No specific treatment exists for West Nile fever beyond supportive care like good nursing and mechanical respirators when needed.

That death rate is half that of West Nile fever, which was 6.7 percent last year, but is still considered relatively high by health officials.

The full clinical spectrum of West Nile fever has not been determined in the United States.

Also, although doctors presume there are no long-term hazards from mild West Nile fever, follow-up studies have not been done to document the point.

The tests came back positive for St. Louis encephalitis, a close relative of West Nile fever.

A prime example is the mosquito-borne West Nile fever, which has spread through the country since it first entered the Americas in 1999.

Dr. Gerberding's agency is responsible for protecting the public's health from infections like West Nile fever.

The number of cases of West Nile fever is expected to rise sharply in the next week and could eventually reach 1,000, federal health officials said yesterday.

An epidemic disease is not required to be contagious, and the term has been applied to West Nile fever and the obesity epidemic, among others.

Then in 1996 the face of West Nile fever changed when the first urban epidemic occurred, in Bucharest, Romania.

But when West Nile fever was detected in New York City, Specialty developed a test for that virus in two weeks.

A large outbreak of an illness in the region in 1941 may have been West Nile fever, but documentation is lacking because diagnostic tests were not available.

Tests have shown that another recipient, a 63-year-old man who lives in Miami-Dade County, Fla., has West Nile fever.

In recent days, the centers asked doctors and hospitals to report possible cases of West Nile fever through blood and organ transplants to their local health departments.

Dr. Gerberding cautioned doctors to consider West Nile fever in patients who experienced headaches and fevers after transfusions.

The test, which also helps doctors diagnose West Nile fever, has led to more milder cases being reported than in the past, Dr. Gerberding said.

West Nile fever (WNF), which occurs in 20 percent of cases, is a febrile syndrome that causes flu-like symptoms.