Image copyright Getty Images Image caption Kinshasa, pictured in 1955, was at the centre of the pandemic, scientists say 
 
 The origin of the Aids pandemic has been traced to the 1920s in the city of Kinshasa, in what is now the Democratic Republic of Congo, scientists say. 
 
 An international team of scientists say a "perfect storm" of population growth, sex and railways allowed HIV to spread. 
 
 A feat of viral archaeology was used to find the pandemic's origin, the team report in the journal Science. 
 
 They used archived samples of HIV's genetic code to trace its source, with evidence pointing to 1920s Kinshasa. 
 
 Their report says a roaring sex trade, rapid population growth and unsterilised needles used in health clinics probably spread the virus. 
 
 Meanwhile Belgium-backed railways had one million people flowing through the city each year, taking the virus to neighbouring regions. 
 
 Experts said it was a fascinating insight into the start of the pandemic. 
 
 HIV came to global attention in the 1980s and has infected nearly 75 million people. 
 
 It has a much longer history in Africa, but where the pandemic started has remained the source of considerable debate. 
 
 Family affair 
 
 A team at the University of Oxford and the University of Leuven, in Belgium, tried to reconstruct HIV's "family tree" and find out where its oldest ancestors came from. 
 
 The research group analysed mutations in HIV's genetic code. 
 
 "You can see the footprints of history in today's genomes, it has left a record, a mutation mark in the HIV genome that can't be eradicated," Prof Oliver Pybus from the University of Oxford told the BBC. 
 
 By reading those mutational marks, the research team rebuilt the family tree and traced its roots. 
 
 Image copyright SPL 
 
 HIV is a mutated version of a chimpanzee virus, known as simian immunodeficiency virus, which probably made the species-jump through contact with infected blood while handling bush meat. 
 
 The virus made the jump on multiple occasions. One event led to HIV-1 subgroup O which affects tens of thousands in Cameroon. 
 
 Yet only one cross-species jump, HIV-1 subgroup M, went on to infect millions of people across every country in the world. 
 
 The answer to why this happened lies in the era of black and white film and the tail-end of the European empires. 
 
 In the 1920s, Kinshasa (called Leopoldville until 1966) was part of the Belgian Congo. 
 
 Prof Oliver Pybus said: "It was a very large and very rapidly growing area and colonial medical records show there was a high incidence of various sexually transmitted diseases." 
 
 Sex and railways 
 
 Large numbers of male labourers were drawn to the city, distorting the gender balance until men outnumbered women two to one, eventually leading to a roaring sex trade. 
 
 Prof Pybus added: "There are two aspects of infrastructure that could have helped. 
 
 "Public health campaigns to treat people for various infectious diseases with injections seem a plausible route [for spreading the virus]. 
 
 "The second really interesting aspect is the transport networks that enabled people to move round a huge country." 
 
 Around one million people were using Kinshasa's railways by the end of the 1940s. 
 
 The virus spread, with neighbouring Brazzaville and the mining province, Katanga, rapidly hit. 
 
 Those "perfect storm" conditions lasted just a few decades in Kinshasa, but by the time they ended the virus was already starting to spread around the world. 
 
 Image copyright SPL Image caption The human immunodeficiency virus (HIV) attacks the immune system 
 
 Prof Jonathan Ball, from the University of Nottingham, told the BBC: "It's a fascinating insight into the early phases of the HIV-1 pandemic. 
 
 "It's the usual suspects that are most likely to have helped the virus get a foothold in humans - travel, population increases and human practices such as unsafe healthcare interventions and prostitution. 
 
 "Perhaps the most contentious suggestion is that the spread of the M-group viruses had more to do with the conditions being right than it had to do with these viruses being better adapted for transmission and growth in humans. I'm sure this suggestion will prompt interesting and lively debate within the field." 
 
 Dr Andrew Freedman, a reader in infectious diseases at Cardiff University, said: "It does seem an interesting study demonstrating very elegantly how HIV spread in the Congo region long before the Aids epidemic was recognised in the early 80s. 
 
 "It was already known that HIV in humans arose by cross species transmission from chimpanzees in that region of Africa, but this study maps in great detail the spread of the virus from Kinshasa, it was fascinating to read." ||||| A new, sophisticated analysis of hundreds of genetic sequences of HIV from different time points and locations adds fascinating insights to the origin of the AIDS epidemic. The study, which appears in this issue of Science (see p. 56), confirms earlier analyses that an HIV-infected person came to what today is Kinshasa around 1920, but it then shows for the first time how the virus went from there to two cities in the southeastern portion of the country, likely aided by the extensive rail system that then existed. The researchers also note that 13 documented cases exist of different simian viruses jumping from chimpanzees, gorillas, and monkeys into humans, but only one—known has HIV-1 group M—sparked a global epidemic. They show that group M and another strain, group O, expanded at the same rate until about 1960, but then group M nearly tripled its rate of spread. Possible reasons include public health campaigns that had contaminated needles and an increase in the number of clients of sex workers. ||||| LONDON Bustling transport networks, migrant labor and changes to the sex trade in early 20th-century Congo created a "perfect storm" that gave rise to an HIV pandemic that has now infected 75 million people worldwide, researchers said on Thursday. 
 
 In an analysis of the genetic history of the human immunodeficiency virus (HIV) that causes AIDS, the scientists said the global pandemic almost certainly began its global spread in the 1920s in Kinshasa in Democratic Republic of Congo (DRC). 
 
 Here, a confluence of factors including urban growth, extensive railway links during Belgian colonial rule and changes in sexual behavior combined to see HIV emerge in Congo's capital and spread across the globe. 
 
 Oliver Pybus, a professor at Oxford University's zoology department who co-led the research, said that until now most studies have taken a piecemeal approach to HIV's genetic history and looked only at certain HIV genomes in particular locations. 
 
 "For the first time, we have analyzed all the available evidence using the latest phylogeographic techniques, which enable us to statistically estimate where a virus comes from," he said. "This means we can say with a high degree of certainty where and when the HIV pandemic originated." 
 
 United Nations AIDS agency (UNAIDS) data show that more than 35 million people worldwide are currently infected with HIV, and some 1.5 million people died of AIDS-related illness in 2013. 
 
 Since the HIV/AIDS pandemic began, it has killed up to 40 million people worldwide. 
 
 The disease is spread in blood, semen and breast milk. No cure exists, but AIDS can be kept at bay for many years in people with HIV who take cocktails of antiretroviral drugs. 
 
 Various strains of HIV are known to have been transmitted from primates and apes to humans at least 13 times in history, but only one of those transmissions - of a strain known as HIV-1 Group M - led to the current human pandemic. 
 
 Pybus said the key questions centered on how this happened. 
 
 "Why did most of (the HIV strains) die out, and why did some of them -- like HIV-2 -- go on to generate local epidemics in Africa, and why did only one go to become a global pandemic?" he said in a telephone interview. 
 
 "To answer that, we needed to try to reconstruct the spread through space and time of the global pandemic strain." 
 
 Pybus and an international team of researchers analyzed HIV-1 group M sequences from a major HIV sequence database and then combining these analyses with spatial and epidemiological data. Their study will be published in the journal "Science" on Thursday. 
 
 Philippe Lemey, a professor at Belgium's University of Leuven who worked on the study, explained that genetic analysis had helped establish the time and place of the pandemic's origins. The team then compared that with historical data "and it became evident that the early spread of HIV-1 from Kinshasa to other population centers followed predictable patterns." 
 
 A key factor, the analysis suggests, was the DRC's transport networks, particularly its railways, which made Kinshasa one of the best-connected of all central African cities. 
 
 "Data from colonial archives tells us that by the end of 1940s over one million people were traveling through Kinshasa on the railways each year," said Nuno Faria of Oxford University, who also worked on the team. 
 
 He said the genetic data showed that HIV rapidly spread across the DRC -- a country the size of Western Europe -- traveling with people along railways and waterways to reach Mbuji-Mayi and Lubumbashi in the extreme south and Kisangani in the far north between the end of the 1930s and early 1950s. 
 
 The team's findings also suggest that along with transport, social changes such as the changing behavior of sex workers and public health initiatives against other diseases that led to the unsafe use of needles may have contributed to turning HIV into a full-blown epidemic. 
 
 "We think it is likely that the social changes around the independence in 1960 saw the virus break out from small groups of infected people to infect the wider population and eventually the world," said Faria. 
 
 (Reporting by Kate Kelland) ||||| A “perfect storm” of urban change that began in 1920s Kinshasa led to the catastrophic spread of HIV across Africa and into the wider world, according to scientists who used genetic sequencing and historical records to trace the origins of the pandemic. 
 
 Though the virus probably crossed from chimpanzees to humans in southern Cameroon years earlier, HIV remained a regional infection until it entered the capital of what is now the Democratic Republic of the Congo. 
 
 From the 1920s until 1960, the pandemic HIV strain – there were others that fizzled out – spread from Kinshasa, crossed borders to other nations, and ultimately landed on distant continents. It has infected nearly 75 million people worldwide to date. 
 
 When the virus arrived, Kinshasa was bustling. It was the largest and fastest growing city in the region with transport links reaching up and down the country. The busy Congo river curved north and east to Kisangani more than 600 miles away. The railway carried scores of workers southeast to Katanga, a mining province reliant on immigrant labour, and on to Lubumbashi more than 900 miles away. 
 
 Records show that by the 1940s, more than a million people a year passed through Kinshasa on the railways alone. By 1960, the rate of new pandemic HIV infections outpaced the growth of the regional population, according to research published in Science. 
 
 While boats and trains spread the virus far, other factors played their part. Records suggest Kinshasa had a relatively high proportion of men and a consequent demand for sex workers. Some doctors may have unwittingly spread the virus further, through unsterilised jabs at sexual health clinics. 
 
 An international team of scientists led by the universities of Oxford in Britain and Leuven in Belgium reconstructed the history of the HIV pandemic using historical records and DNA samples of the virus dating back to the late 1950s. The DNA allowed them to draw up a family tree of the virus that traced its ancestry through time and space. Using statistical models they could push farther back than the 1950s and locate the origin of the pandemic in 1920s Kinshasa. 
 
 People with HIV in central Africa at the time did not have specific symptoms that would have been written down in their medical records. The virus causes the immune system to collapse, leaving people open to all manner of infections. “For an epidemic like HIV where we’re trying to track back to before it was even discovered, genetics is the only source of information we have,” said Oliver Pybus, an evolutionary biologist at Oxford University and senior author on the study. 
 
 The genetic data suggests that pandemic HIV spread rapidly through the Democratic Republic of the Congo, a country the size of western Europe. From the late 1930s to the early 1950s, the virus spread by rail and river to Mbuji-Mayi and Lubumbashi in the south and Kisangani in the north. There the virus took hold and formed secondary reservoirs from where it spread to countries in southern and eastern Africa. 
 
 At first, HIV was an infection confined to specific groups of people. But the virus seemed to break out into the general population and spread around the world after what was then known as the Republic of the Congo achieved independence in 1960. 
 
 “Parts of the story can only be suggestive. Without a time machine it’s very difficult to prove causality. But we can be fairly sure we have the time and place where this happened,” Pybus said. “It seems a combination of factors in Kinshasa in the early 20th century created a perfect storm for the emergence of HIV, leading to a generalised epidemic with unstoppable momentum that unrolled across sub-Saharan Africa.” 
 
 Different strains of HIV have almost certainly jumped from apes to humans – through hunting or handling bushmeat – scores of times throughout history. Only a dozen or so incidents have left their traces in the DNA of HIV strains around today. Some outbreaks infected hundreds of thousands of people in Africa but went no further. Only one, known as HIV-1 group M, went pandemic. 
 
 “We all want to know how this happened, how we got here,” said Pybus. “We wanted to answer the question: why did this one turn into a pandemic?”
– More than half a century before AIDS even had a name, the HIV virus was already spreading across Africa via the city now known as Kinshasa, researchers say. In the 1920s, the city was Leopoldville, capital of the Belgian Congo, and scientists say a "perfect storm" of conditions caused the pandemic to emerge, the BBC reports. The fast-growing city was full of transient male workers who visited prostitutes, researchers say. Sexually transmitted diseases were rife, and unsterilized needles at health clinics also helped spread the disease, according to a study published in the journal Science. Researchers studied HIV's genetic code as well as historical documents. The researchers note that strains of HIV have been transmitted from primates to humans at least 13 times, but only one has caused a global epidemic. They believe a key factor in the 1920s spread was the Congo's booming transport network early last century, which saw a million people travel through Kinshasa every year on the railways alone, reports Reuters. Researchers say the disease was at first confined to specific groups of people, but it started to spread into the general population and around the world after 1960, the Guardian reports. It took genetic research to track the pandemic back to its source because the disease hits the immune system, meaning victims died from any of a wide variety of infections, making the spread of HIV before it was identified hard to track through medical records, says the lead researcher, an evolutionary biologist. (HIV has infected 75 million people, but UN officials say real progress is being made and the epidemic could be over by 2030.)