Spain is of the most wide-ranging HIV exporters
Scientists who have mapped HIV's spread across Europe say holidaymakers infected abroad are largely to blame.
By analysing samples from 17 European countries, the international team tracked the movement of the virus around the continent.
Their map shows Greece, Portugal, Serbia and Spain are big HIV exporters, with many tourists to and migrants from these countries leaving with the virus.
The UK is an exporter and importer, Retrovirology journal says.
The same is true of Israel, Norway, the Netherlands, Sweden and Switzerland, while countries like Austria, Belgium, Denmark, Germany and Luxembourg are largely importers of HIV, the researchers say.
In Poland, HIV is contained but is spread among its inhabitants because of injecting drug-users, the research group found.
To construct their map, the researchers looked at the most common type of HIV circulating in Europe, known as HIV-1 subtype B.
They tracked its migration by creating a family tree for the virus, looking at detailed genetic characteristics that reveal how the virus has been evolving over time.
DIRECTION OF HIV SPREAD
Exporters: Greece, Portugal, Serbia and Spain
Both exporters and importers: Israel, Norway, the Netherlands, Sweden, Switzerland and the UK
Importers: Austria, Belgium, Denmark, Germany and Luxembourg
Lead author Dimitrios Paraskevis, of the University of Athens, said: "Popular tourist destinations like Greece, Portugal and Spain probably spread HIV with tourists infected during their holidays."
In the case of Serbia as an exporter, it is most likely down to its inhabitants travelling to other countries and carrying the virus with them, he said.
However, the researchers also noted that although, overall, Serbia is more of an HIV exporter than an importer, in the league of the HIV-exporting countries, Serbia does not rank very high. Indeed, some of the countries deemed bidirectional (both importers and exporters) ranked higher.
He said: "To a large extent HIV spread within Poland is due to injecting drug-users, who make up around half of the HIV-infected population.
"Viruses move around with travellers - thus health programmes within countries should not only target the national populations, prevention efforts must also be aimed at migrants, travellers and tourists - who are both major sources and targets of HIV."
Rowan Harvey, of the Terrence Higgins Trust, said: "HIV isn't constrained by borders, it's a global epidemic and there are bound to be patterns of transmission between countries.
"Tourists travelling abroad should definitely pack condoms, but people should also be aware that HIV is at its highest level in the UK as well.
"To protect yourself from HIV and other sexually transmitted infections, safer sex is essential both at home and abroad."
Adapted from the researchers' original map to show the main exporter countries