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A new genetic study confirms theories that the global epidemic of HIV and AIDS started in New York around 1970, and it also clears the name of a gay flight attendant long vilified as being "Patient Zero."

Researchers got hold of frozen samples of blood taken from patients years before the human immunodeficiency virus (HIV) that causes AIDS was ever recognized, and teased out genetic material from the virus from that blood.

They use it to show that HIV was circulating widely during the 1970s, and certainly before people began noticing a “gay plague” in New York in the early 1980s.

“We can date the jump into the U.S. in about 1970 and 1971,” Michael Worobey, an expert on the evolution of viruses at the University of Arizona, told reporters in a telephone briefing.

“HIV had spread to a large number of people many years before AIDS was noticed.”

Their findings also suggest HIV moved from New York to San Francisco in about 1976, they report in the journal Nature.

A laboratory technician examines blood samples for HIV Eliseo Fernandez / Reuters, file

“New York City acts as a hub from which the virus moves to the west coast,” Worobey said.

“HIV had spread to a large number of people many years before AIDS was noticed.”

Their findings confirm widespread theories that HIV first leapt from apes to humans in Africa around the beginning of the 20th century and circulated in central Africa before hitting the Caribbean in the 1960s. The genetic evidence supports the theory that the virus came from the Caribbean, perhaps Haiti, to New York in 1970. From there it spread explosively before being exported to Europe, Australia and Asia.

HIV now infects more than 36 million people worldwide. About 35 million have died from AIDS, according to the United Nations AIDS agency. Two million are infected every year and more than 1 million died of it last year.

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In the United States, more than 1.2 million people have HIV, and about 50,000 people are newly infected each year. More than two dozen medications now on the market can keep infected people healthy, and can prevent infection, but there is no vaccine and there is no cure.

Researchers now know a lot about HIV and how it attacks the body’s immune cells, gradually destroying the immune system. It takes about 10 years or so for untreated and undiagnosed patients to realize something is badly wrong as they develop AIDS-defining illnesses such as Kaposi’s sarcoma, a rare form of cancer, or pneumonia, or as they succumb to tuberculosis.

It infects men, women and children alike, is passed to newborns from infected mothers, and spreads like wildfire in infected needles shared by drug users.

But in the early 1980s in New York and San Francisco, all people knew was that something mysterious was killing gay men.

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“Death and dying was such a part of our daily lives."

"In New York City, the virus encountered a population that was like dry tinder, causing the epidemic to burn hotter and faster and infecting enough people that it grabs the world's attention for the first time,” Worobey said in a statement.

“Death and dying was such a part of our daily lives,” said Sean Strub, a writer and activist who has been HIV positive since the 1980s.

Related: Drugs Make HIV Hard to Transmit

Once the virus was identified, researchers could start working on treatments and diagnostic tests. And specialists like Worobey could look at the genetics of the virus itself to find its origins.

But no one had samples of HIV infected blood from before the 1980s.

Worobey’s team got nine samples and sequenced the RNA from eight of them. It wasn’t easy to get and was damaged, so they used new techniques similar to those used to reconstruct DNA from ancient Neanderthal remains.

The different mutations in the genetic material — DNA from people, RNA from the virus — can provide a genetic clock. The more diversity, the more time a virus has been circulating and acquiring small mutations.

"That information is stamped into the RNA of the virus from 1970," Worobey said.

"Our analysis shows that the outbreaks in California that first caused people to ring the alarm bells and led to the discovery of AIDS were really just offshoots of the earlier outbreak that we see in New York City."

Related: AIDS Activists Still Fight Stigma

Other experts not involved in the research called it impressive.

“For the first time, they found and characterized samples that had a date on the tube that preceded the discovery of the disease,” said Dr. Beatrice Hahn, who studies the genetics and evolution of HIV and other infectious agents at the University of Pennsylvania.

“They had tubes with 1978, 1979 written on them,” she added. “That was something very powerful.”

If someone directly infects someone else with a virus, usually it’s easy to tell from the genetic sequence. Samples taken from each person will be very close genetically, if not identical. Because the HIV viruses taken from the different samples showed many different mutations, that shows HIV had been infecting many people for quite a time.

Related: Black Gay, Bisexual Men Have 50 Percent Risk of HIV

“This means this virus was in this country way before the disease was recognized,” Hahn said.

“By the time people discovered the disease as a completely new entity that came out of the blue and killed people left and right for really unknown reasons, the disease had already been in this country for 10 years prior. That’s a little sobering.”

Using their genetic clock, Worobey’s team showed the virus infecting men in the 1970s in San Francisco had fewer mutations. “Compared to NYC, the San Francisco epidemic in 1978 appeared to have been established more recently,” they wrote.

“This suggests that the bulk of the HIV-1 infections in San Francisco in 1978 traced back to a single introduction from New York City in around 1976.”

“I think in many ways this paper may be one of the last pieces, if not the last piece in the puzzle."

The Worobey team also sequenced samples of virus taken from Gaetan Dugas, a Canadian flight attendant named as "Patient Zero." Dugas died in 1984 and stunned researchers when he told them he’d had about 250 sexual partners a year between 1979 and 1981, although it later became clear that was not uncommon.

Related: Southern, Gay Men Have High HIV Rate

The sequences make it clear he was a victim of an epidemic that had already been raging, and not its originator, Worobey said.

"It’s shocking how this man’s name has been sullied and destroyed by this incorrect history," said Peter Staley, a former Wall Street bond trader who became an AIDS activist in New York in the 1980s.

“He was not Patient Zero and this study confirms it through genetic analysis,” Staley told NBC News.

“No one should be blamed for the spread of viruses,” Worobey said.

He said he hopes the techniques his team used can help explain the patterns of other epidemics, such as Ebola, SARS and Zika.

“I think in many ways this paper may be one of the last pieces, if not the last piece in the puzzle,” Hahn said. “I am a believer of putting the last nail into the coffin. And I believe that is what was done here."