Health officials have identified a new possible target in their fight against West Nile virus as it spreads across the country.

They say a woman may have gotten the virus through blood transfusions, and four people possibly became infected after receiving her organs following her death.

One of the organ recipients has died, and three were hospitalized with symptoms associated with West Nile, although tests to determine if they were infected with West Nile through the transplants are still being conducted.

"We've known for some time that there is a theoretical possibility that people can get this through blood or organ transplants," said Tom Skinner, a spokesman for the Centers for Disease Control and Prevention. "It's highly unusual but it's certainly possible."

American Red Cross spokeswoman Trudy Sullivan acknowledged that no test exists to screen blood for West Nile, but she said the blood supply is safer than ever and anyone showing symptoms of the virus would be turned away.

"Yes, the screening process doesn't cover everything, but if you've even got flu-like symptoms, you will be turned away," Sullivan said. "We have a number of different layers of safety."

West Nile infections can be hard to spot, though. Only about 1 percent of those infected show symptoms, with those with weakened immune systems most susceptible, according to the CDC. So far this year, 638 people in 28 states have tested positive for West Nile virus and 31 have died.

All the previous cases had been blamed on mosquito bites, but health officials now fear four people may have been infected with West Nile after receiving the kidneys, heart and liver of a woman who died in Georgia last month after a car accident, the CDC said Sunday.

Officials say the woman may have been infected already or may have gotten West Nile through blood transfusions in the emergency room.

The Atlanta-based CDC is trying to trace donors who contributed the transfused blood, the blood products made from the donations, and any other patients who may have received blood or blood products from the donations.

The Food and Drug Administration issued an alert to blood banks two weeks ago to exercise extra caution when screening donors, Dr. Jesse Goodman of the FDA said.

"We have been very active and tried to anticipate the possibility of something like this," Goodman said.

Three of the four patients developed symptoms of encephalitis, the inflammation of the brain and central nervous system, which is the most serious consequence of West Nile virus.

One of the four has died in Atlanta, said Dr. James Hughes, director of the CDC's National Center for Infectious Disease. Standard pathology tests from an autopsy confirmed the recipient had encephalitis. Tests are ongoing to see if the recipient was infected with West Nile, which causes encephalitis.

Another recipient, from Jacksonville, Fla., showed symptoms of encephalitis Sunday, said Dr. John Agwunobi, the Florida Secretary of Health.

CDC officials say they're sure the man didn't contract the disease from a mosquito, said Mary Jo Trepka, epidemiology director with the county health department.

Samples from the four transplant recipients were sent to the CDC's lab in Fort Collins, Colo., Hughes said. Test results are expected within the week.

There is no test yet that can quickly or accurately identify the presence of the West Nile virus. Patients are diagnosed based on their immune response to the virus.

However, researchers at the CDC are trying to find a way that will cut down the time from when infection occurs and when a response to the virus can be measured, the CDC said. It now takes about 15 days.

Officials say they remain optimistic that there is a low chance West Nile can spread through blood because there have been no confirmed cases to date.

There are also no known cases of person-to-person transmission of other diseases in the same family as West Nile called the arboviral encephalitides: St. Louis encephalitis, La Crosse encephalitis and Eastern and Western equine encephalitis.

However, there have been cases of other mosquito-borne diseases being passed by blood transfusion or transplant. Last year, a CDC review of blood-donation problems turned up 93 patients who contracted malaria after blood transfusions.

Cases have also been reported in which dengue, another mosquito-borne disease, was transmitted to a health care worker by a needle-stick and between siblings after a bone-marrow transplant.

Sullivan said the blood supply is safer than it's ever been, and that her organization is "focused on allaying any fears or concerns about the safety of the blood supply for both donors and recipients."

Every unit of blood donated goes through up to 12 tests to ensure patient safety, including tests for HIV and hepatitis C, Sullivan said.