In the tortuous mythology of the AIDS epidemic, one legend never seems to die: Patient Zero, a.k.a. Gaétan Dugas, a globe-trotting, sexually insatiable French Canadian flight attendant who supposedly picked up H.I.V. in Haiti or Africa and spread it to dozens, even hundreds, of men before his death in 1984.

Mr. Dugas was once blamed for setting off the entire American AIDS epidemic, which traumatized the nation in the 1980s and has since killed more than 500,000 Americans. The New York Post even described him with the headline “The Man Who Gave Us AIDS.”

But after a new genetic analysis of stored blood samples, bolstered by some intriguing historical detective work, scientists on Wednesday declared him innocent.

The strain of H.I.V. responsible for almost all AIDS cases in the United States, which was carried from Zaire to Haiti around 1967, spread from there to New York City around 1971, researchers concluded in the journal Nature. From New York, it spread to San Francisco around 1976.