How HIV spread across the West

North America was seen to be spreading the virus much more than importing it by transmitting it out of the continent and into Western Europe on multiple occasions.

"It wasn't just a random transition or single point of introduction; it was happening constantly," Magiorkinis said. He stressed that these are movements of the virus, not migrants. "It's not immigrants causing this, as people going on holiday can get infected."

This continuous flow is, in part, thought to be due to certain countries being more influential than others, such as those in North America. A country is considered influential by having many connections to other parts of the world, as once any virus enters, it can then easily spread.

"It was only when it entered the USA that it became a pandemic," Magiorkinis said.

In contrast, Europe was found to have absorbed constant flows of the virus from multiple regions of the world, including the United States.

"Within Europe, three countries had higher connections: the UK, France and Switzerland," Magiorkinis said. These connections saw these countries both releasing and absorbing the virus, but with greater numbers coming in, including from the United States.

"The critical issue is population-mixing and the fact people acquire, or spread, viruses associated with travel," said Chris Beyrer, professor of public health and human rights at the Johns Hopkins Bloomberg School of Public Health. "There's a long history of blame for the spread of the epidemic ... but you have to improve control and treatment."

Multiple studies have shown the benefits of antiretroviral treatment in preventing transmission of HIV as levels of the virus can become undetectable once on treatment.

After arriving in Europe, the virus soon spread within the continent but with a clear segregation between Eastern and Western Europe as each side evolved its own pockets of the epidemic. In Western Europe (as well as North America), HIV mainly affects gay communities, causing concentrated epidemics. In the UK, more than half of new HIV infections occurred among gay men in 2014, despite them making up an estimated 2% to 3% of the male population. But in Eastern Europe, the virus mainly infects injecting drug users, and transmission dynamics are quite different, as the virus is not spread sexually.

This segregation of epidemics occurred until the collapse of the Soviet Union in the 1990s, when movement -- and transmission --promptly changed.

"When the Soviet Union fell, we can see the virus in that area spilled over into Western Europe, probably due to migration," Magiorkinis said.

The study was hypothetical and based on genetic analysis, so they cannot be proved. Beyrer also stresses that the Oxford team's findings have their limitations: "It's a huge pandemic, and huge numbers are not provided in these samples. You couldn't infer much about the rest of the world from this study."

But the burden in Eastern Europe today has public health teams concerned, as this is one of only two regions in the world today where numbers infected with HIV are rising, and any insight into transmission can be used to better target resources.

"The future of HIV transmission in Europe is the East," Beyrer said.

The study also reveals the ease with which HIV and other viruses can spread across the world, calling for more global solutions to the problem.

"Public health responses to HIV must be thought of as a global response, because you cannot stop movement between countries," Magiorkinis said. "It's crucial to build health systems that provide access to treatment faster and reduce transmission."