We’ve known for years that HIV can be traced back to 1920s Africa. But how and when did the virus take hold in the United States? To find out, a team of researchers led by evolutionary biologist Michael Worobey, PhD, looked at blood samples taken from gay and bisexual men in 1978 and 1979 as part of a hepatitis B study.

Turns out that HIV moved from Africa to the Caribbean by 1967 and from there to New York City by 1971. Another single virus reached San Francisco about three years later. By the end of the 1970s, the virus had spread enough in New York to show genetic diversity.

Worobey also looked at a blood sample from Canadian flight attendant Gaëtan Dugas, a.k.a. Patient Zero, the man blamed by journalists in the ’80s for spreading AIDS across America. On the family tree of the virus, Dugas fell in the middle, not at the beginning. “Beliefs about Patient Zero,” Worobey concludes, “are unsupported by scientific data.”