HIV broke into the public imagination in the early 1980s after a cluster of young gay men in San Francisco, LA, and New York City developed unexplainable cases of skin cancer and pneumonia. At first glance, doctors suspected these men were abusing party drugs, but they quickly became aware of a much more worrying prospect: a transmittable virus.

The virus didn’t even have a name yet, but slowly mounting concern from the public was pushing scientists to figure out where this emerging illness was heading. To do so, they first wanted to work out where, or who, it came from. 

In 1981, investigators from the Centers for Disease Control and Prevention (CDC) interviewed many of the young men living in the Los Angeles area with the mysterious illness in the hopes of finding some kind of link. When asked about their sexual history, one name repeatedly popped up: Gaëtan Dugas.

During his regular travels as a flight attendant in the 1970s and early 1980s, this sexually-voracious Canadian man was believed to have slept with thousands of men, potentially spreading the virus to hundreds of people across North America and the wider world. It was reported at least eight of the first 19 HIV cases reported in Los Angeles alone had sexual relations with Dugas or one of his previous sexual partners.

His face and the tagline “PATIENT ZERO” was later splashed across newspapers and TV reports, often alongside accusations that he recklessly – even maliciously – helped to bring the virus across the Atlantic and began the domino effect across the world. 

However, this idea of Dugas as “patient zero” is a total myth. A Nature study of his HIV genome in 2016 revealed that it was typical of strains of the virus within the US at the time and was not the root of the virus' diversification in North America. Dugas, who died of AIDS complications in 1984, was absolved of being “patient zero”.

“Gaétan Dugas is one of the most demonized patients in history, and one of a long line of individuals and groups vilified in the belief that they somehow fuelled epidemics with malicious intent,” said Dr Richard McKay from Cambridge University's Department of History and Philosophy of Science.

“In many ways, the historical evidence has been pointing to the fallacy of Patient Zero for decades,” he added. “We now have additional phylogenetic evidence that helps to consolidate this position.”

The real story is much more global and fiddly. Scientists now know that there are two main types of the human virus: HIV-1 and HIV-2. The most prevalent and most infectious type, HIV-1, is then broken down into groups M, N, O, and P. Group M is the most common group and it’s responsible for 90 percent of HIV-1 infections worldwide. By looking at its genetic building blocks and noting the patterns of mutations, scientists can work backward to figure out its family tree and show how it first emerged in people.

It’s clear that the HIV-1 group M virus settled and spread in the US between 1970 and 1971 after the virus jumped from the Caribbean to New York City, eventually spreading to San Francisco by 1974. HIV emerged in the Caribbean when it was first carried to Haiti in 1967 from the Democratic Republic of the Congo. The oldest known sample of HIV-1 comes from a blood sample collected from a man in 1959 who was living in Léopoldville, now Kinshasa, in the Democratic Republic of the Congo. However, this is simply the earliest sample that has been kept and tested – the story still goes back further.

Recent research has shown that the city of Kinshasa played a huge role in the early spread of HIV from at least 1920 onwards. Over the following 40 years, the virus quietly lurked in this bustling port town, until it met the perfect storm – new technology and colonialism were connecting the world, for better or for worse, while changing social attitudes and prolific prostitution meant that sex was everywhere.

Around this time, a handful of people from outside of sub-Saharan Africa are believed to have contracted the virus. Most notable is Arvid Noe, a Norwegian sailor who traveled to Africa on numerous voyages in the early 1960s. Noe, along with his wife and daughter, died of AIDS complications in 1976, some of the earliest documented HIV/AIDS-related deaths.

The virus had first journeyed to Kinshasa in the 1920s from Cameroon, most likely down through the trade routes of the Sangha River and the Congo River. Here, in the chimp-filled rainforests of Cameroon, it’s believed that the HIV virus was transferred to humans by a non-human primate. We think this because chimp poop from this area contains a mutated form of the simian immunodeficiency virus (SIV) that’s suspiciously similar to early forms of HIV. 

The first person to get the human virus was most likely a bushmeat hunter somewhere in Cameroon at some point between 1900 and 1920. They will probably never be identified because of the lack of documents from the time and the absence of any firsthand evidence. The most credible theory says that this unknown person cut themselves while butchering chimpanzee meat that was infected with a mutated strain of SIV. The urban myth that this initial transfer was through a human and a chimp having sex is not taken seriously by any scientists.

However, researchers do know that this jump between species has actually occurred multiple times. HIV-1 Group M and N stemmed from SIV found in chimps from southeastern Cameroon. HIV-1 Group N came from western gorillas and HIV-2 came from sooty mangabeys, a smaller monkey from tropical West Africa. So, this idea of a single "patient zero" is a largely fruitless concept, both socially and scientifically.

What’s perhaps most remarkable about HIV’s short history is that its transmission requires direct contact with an infected person’s blood. There’s next to no risk of a casual transmission from a handshake, a cough, or a toilet seat. Despite this, within just over 100 years, this virus has managed to spread from a rural rainforest to more than 70 million people from every corner of the world. 

Only by tracing back to the origin of the virus can you fully understand the unique and unlikely set of circumstances that snowballed into one of the biggest health challenges of the modern era. Thankfully, this journey appears to be slowly but surely coming to an end.