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The American Red Cross, which provides about 40% of the U.S. supply, last month began accepting donors previously deferred because of the risk of mad cow disease, formally known as variant Creutzfeldt-Jakob disease, or vCJD.

Invariably fatal, with an incubation period that can last years or even decades, the emergence of vCJD in humans in the late 1990s alarmed officials responsible for the safety of the blood supply, said Dr. Rita Reik, chief medical officer for OneBlood, a collection center in Florida.

Bans remain in place for people who are suspected of having vCJD or related diseases, those who have a blood relative with a related disease and those who received pituitary human growth hormone or a certain type of brain tissue transplant from cadavers.

In the U.K., five cases of vCJD were transmitted by blood transfusions, according to the National Health Service.

Its human form - vCJD - has been attributed to 178 deaths.