European ministers must act now to help new EU member states to tackle growing rates of HIV and Aids, experts say.
1.5m people in Eastern Europe and Central Asia have HIV
A conference in Dublin this week will hear how countries such as Latvia, Lithuania and Estonia have some of the fastest growing epidemics in the world.
Experts say the 10 new member states will need financial and technical help to prevent the spread of HIV/Aids.
Dr Peter Piot, head of UNAIDS, called for improved prevention and treatment, saying: "There is no time to waste."
He added: "Of all the social and political challenges facing an expanded European Union, Aids is one of the greatest, requiring determined and sustained action now.
"Eastern Europe and Central Asia have the fastest growing Aids epidemics in the world, with rapid cross-over from high-risk groups into the general population.
"Furthermore, rates of HIV infection continue to spread in Western Europe."
Cyprus and Malta plus the former communist states of the Czech Republic, Slovakia, Poland, Lithuania, Latvia, Estonia, Hungary and Slovenia will become members of the European Union in May this year.
The threat of the growing HIV/Aids epidemic in Eastern Europe will be discussed at a special conference of health ministers and HIV/Aids experts in the Irish Republic, which currently holds the EU presidency.
Dr Piot added: "European ministers must urgently scale up and roll out effective HIV prevention and treatment programmes.
"Given that the EU will form the biggest trading bloc in the world, covering more than 500m people, it is in the EU's best interest to prevent the Aids epidemic from crippling Europe's social and economic development."
While Eastern Europe is seeing rising rates of HIV and Aids, in Western Europe sexually transmitted infection rates are increasing.
This indicates more people in Western Europe are having unprotected sex, which raises their risk of becoming infected with HIV.
Lars Kallings, the UN Secretary-General's Special Envoy for HIV/Aids in Eastern Europe, said: "The enlarged EU and its neighbours could rapidly be faced with a more vigorous phase of the epidemic unless political leaders transform their verbal commitments into concrete action on the ground,"
Dr Lee Jong-wook, Director-General of the World Health Organization, added: "Europe cannot divide over the issue of Aids treatment, and only provide treatment in the richer countries.
"Treatment should be a right for all, including for sex workers and injecting drug users."
Over 1.5m people in Europe and Central Asia are now living with HIV, compared to only 30,000 in 1995.
A large number of the cases are among young people who use drugs and engage in unsafe sex.
Experts say there is evidence that people are increasingly having sex at a much younger age without protection.