 Where does HIV come from? For some of us who remember the sudden emergence of HIV into the popular culture and awareness of the US in the early 1980s, HIV appears to have come from nowhere. Rumors swirled about where it came from, how it came to infect people from all walks of life around the world. It's only now, after over 25 years of research and discovery, that we can finally start to say with some confidence where HIV came from. HIV is a lentivirus on the basis of its genome and life cycle. It's not all that unusual for the lentiviruses. It infects immune cells, primarily CD4 positive T cells and macrophages. The other lentiviruses pursue similar, if slightly different, life cycles. There are lentiviruses that infect cats, goats, horses, sheep, cows and monkeys. There's even a puma lentivirus. Not all of them cause disease under certain conditions. For example, the simian immunodeficiency virus strain that infects African green monkeys doesn't usually cause much disease to its host. The rhesus macaque also has a strain of SIV that causes little or no disease. However, if you take a rhesus macaque, a monkey from Asia, and infect it with the SIV from the African monkey, or if you infect the African monkey with the Asian virus, the result is called simian AIDS, and it parallels what we see in humans. This observation led scientists to suspect that HIV has not been a human virus for very long. If it was a virus with a lot of history with humans, it's unlikely that it would still be so pathogenic. Where could HIV have come from? We sequenced the genome of thousands of strains of HIV, and while there was an incredible diversity of sequences at certain locations, they all had a strong homology to various SIV strains. We usually break up the strains of HIV into two main groups, the HIV-1 and HIV-2 species. The closest relative, according to sequence variations, for the HIV-1 strain was the SIV that infects chimpanzees. The closest relative of HIV-2, a viral species largely found in Africa, appears to be the SIV found in Suri Manga Bays. Africa possesses far more of the subgroupings of HIV than any other continent. Europe and North America are dominated by a very narrow range of viral types, the group M subtype B. This is suggestive, but not confirmatory, of an African origin for the parent strains. We know that SIV and HIV are both spread in the same ways, sex and blood transfers. Blood-to-blood exposures, however, are 900 times more likely to result in infection than sex contact. It's therefore likely that the transmission from primate to human, called a zoonotic event, was the result of hunters trapping, butchering, and eating chimps. The practice of hunting primates for what is called bushmeat is well documented in tribal groups in certain areas. So we have a strong suggestion of where to look, African monkeys. In fact, evidence of viral evolution suggested a certain region of western equatorial Africa. It was here that a colony of chimpanzees were found with a close relative of the ancestral strain of HIV. Based on mutation and divergence rates, we can place at least one transfer of virus from chimp to human at or around 1931 in the country now known as the Democratic Republic of Congo. There is some evidence to suggest that previous transfers of chimp to human may have occurred as far back as the 1880s, but it's not yet clear if these prior transfers had any ancestry to the current strains. The first clear case of verified infection in a human sample was in the 1960s. So the initial expansion was probably in the mid to late 50s, but may have been earlier. Since the human evidence is not available to us, that's a mystery that may never be unraveled. What was going on in the 1950s in Africa? Urbanization for one. The cities of equatorial Africa experienced a population boom after World War II, along with colonial expansion. The world wanted the raw resources in Africa and needed local labor to extract it. This led to a disruption of traditional lifestyles and the rise of a post-industrial culture in a previously agrarian society. Prostitution and drug use may have increased as people left their towns and villages to find profitable work in the growing cities. They probably left their families in close communities behind, and many were no doubt lonely and isolated in this new cultural setting. It could be said that it was not African culture that led to the spread of HIV, but the effect of colonialism and industrialization in an area that had frequent contact with our close primate relatives. Previous zoonotic transfers of HIV, and it's likely that there were many over the centuries, from prey to hunter, were previously isolated by the low infection rate from anything but blood-blood contact. It was the addition of injectable drugs, prostitution, and urbanization that probably allowed HIV to escape the small region in which it frequently emerged. It appears that the next link in the chain was either to Haiti by direct immigration, or directly to the US. Or it might have been spread to Europe by sailors on cargo ships and then to North America. The data is a bit spotty, but the emergence in the major cities of Europe was slightly after the disease was documented in New York and San Francisco. Asia and Russia may have been infected by a second wave of emergence, as their subtype profiles are markedly different than the European and North American subtypes. South America, Australia, and Northwest Asia are a possible third wave emergence. The popularized idea of the Canadian flight attendant as patient zero is very misleading. He was certainly not the first case outside of Africa or in North America, but he was the index case in a large cluster of 248 patients in 1983. The other origin rumors that are very prevalent are the man-made pandemic theories. For example, the oral polio virus vaccine transmission theory, which has no basis in fact, but it satisfies the need some people have for a good narrative with a shadowy, evil agency acting behind the scenes. The OPV hypothesis, advanced by BBC correspondent Edward Hooper and documented in his book, The River, suggests that an SIV-contaminated chimpanzee culture was used to prepare the oral polio vaccine given in the late 50s and early 60s. His theory, which was effective at selling his book and ignited the imagination of conspiracy theorists, proposed that this SIV-contaminated vaccine culture humanized the Simeon virus. But as the evidence shows, HIV had emerged in its modern form prior to the polio vaccine era. The frozen vaccine lots of that time period were thawed and tested for the presence of either SIV or HIV by an impartial third party. It's also completely implausible that HIV is an engineered virus or bioterror agent. The state of molecular biology in the 1950s was rudimentary. The capability did not exist at the time to make the precise molecular changes needed to generate HIV from SIV. Even crude techniques like co-culturing of the Simeon virus with known human pathogens like influenza would have been unsuccessful. HIV shows all the hallmarks of a zoonotic jump, not design. For example, look again at the distribution of HIV claims around the world. Note that there is profoundly more diversity in Africa. How could we account for that if every continent had been infected at roughly the same time by the same virus? Here are the figures for the number infected. Why would Africa have such a high prevalence and only in certain areas? We could implicate cultural differences, but then why is there such a gradient of infection across African nations of similar culture and population composition? If all these populations were simultaneously infected, why would such large differences be apparent? As a scientist, I remain open to new evidence on this topic. However, accusation is not the same as evidence. Until the conspiracy theorists of the world do some real empirical testing, their claims can be dismissed out of hand. I hope this has been informative. There's one concluding thought I'd like to leave you with. There's so much information on the internet, which isn't based in fact, may be medically dangerous or based on a paranoid fantasy. Here's a general rule. Call it concordance's rule. Never take medical advice from the internet. Ask a licensed qualified doctor what the best plan is for your health. Discuss with him or her any concerns you may have and radical life changes you want to try. Listen to their advice. Your local doctor can help you sort the facts from the fiction and keep you from doing harm to yourself or others. Do it for the people you love and do it for yourself. Thanks for watching.