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HIV was born in a 'perfect storm' of bustling trade routes and migrant workers in 1920s central Africa, a study has claimed.

Scientists from Oxford University and Belgium analysed the genetic history behind the virus, which killed 1.5million people last year alone, and painstakingly traced it back to the city of Kinshasa.

Now the capital of the Democratic Republic of Congo, Belgian colonial rulers had made Kinshasa a thriving trade hub with modern railways which allowed the virus to travel thousands of miles.

Melting pot: Today's HIV pandemic had its roots in 1920s Kinshasa, a study has claimed. Now the capital of the Democratic Republic of Congo, the city (pictured in 1955 when it was called Leopoldville) had busy rail routes

That combined with urban growth, migrant workers and changes to sexual habits and the sex trade, formed a 'perfect storm' which allowed the virus to spread unnoticed until it was too late to stop it.

The researchers combined genetic analysis with statistical data on historical factors, such as how the population of central Africa was distributed at the time.

They focused on HIV-1 Group M, which was just one of several strains which were passed at some point from primates into humans.

Unlike the other strains, HIV-1 Group M proved more devastating, and is the strain which affects most of the 35million people affected around the world today.

Despite Kinshasa being a melting-pot, HIV was seemingly slow to spread beyond the city's borders.

It took as long as 30 years for it to crop up in three other cities in the vast nation - Mbuji-Mayi, Lubumbashi and Kisangani, as far as 1,000 miles away.

How times change: Kinshasa today. The virus has long since spread worldwide, killing up to 40million people

It stayed in the Congo - which is the size of all of western Europe combined - until spreading to the U.S. and around the world from the 1960s onwards, before the number of cases exploded in the 1980s.

Because HIV exists 'silently' before causing acquired immunodeficiency syndrome (AIDS), it was only identified as the virus behind the condition in 1983.

By then thousands of people had already died of AIDS-related illnesses.

Since then scientists have been engaged in a long battle to improve treatment with antiretroviral drugs, which slow the course of HIV and allow some patients to live healthily for decades. 

Deadly: HIV seen in false colour through a microscope. Only one strain turned into a pandemic

Thanks to their efforts and education about safe sex, AIDS deaths have slowly been declining since 2005 - when they peaked at 2.3million worldwide - to 1.5million last year. 

Professor Oliver Pybus of Oxford University, a leader of the new study in the journal Science, said it was the most comprehensive genetic analysis so far of HIV.

He said: 'For the first time, we have analysed all the available evidence using the latest phylogeographic techniques, which enable us to statistically estimate where a virus comes from.

'This means we can say with a high degree of certainty where and when the HIV pandemic originated.' 

Professor Philippe Lemey of Belgium's University of Leuven said 'it became evident that the early spread of HIV-1 from Kinshasa to other population centres followed predictable patterns.'

A key factor, he added, was the fact Belgian colonisers had made Kinshasa one of central Africa's best-connected cities.

Nuno Faria of Oxford University added: '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. 

'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'.

Since the HIV/AIDS pandemic began it has killed up to 40million people around the world. It is spread in blood, semen and breast milk, often via unsafe sex or contaminated needles.  

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