Scientists believe migratory birds first brought the virus to the Balkans area from Russia. The transfer occurred while they travelled south along the Pontic migratory route, which stretches south from northern Russia and Europe and passes along the Black Sea's western coast.

In Turkey, four children died of H5N1 last month and authorities have culled 1.3 million birds in domestic flocks to halt its spread.

Romania, home to Europe's largest wetlands in the Danube delta, has detected cases in birds in 26 villages, while to the north Ukraine officials have destroyed hundreds of thousands of domestic birds after numerous outbreaks there.

Croatia has also detected H5N1 in wild swans.

Health experts agree that a big pandemic of something is overdue. In the last century there were three major pandemics of influenza - the 1918 flu, which killed anywhere between 20 million and 100 million people, depending on the estimate, the 1957 flu, which killed about 4 million, and the 1968 flu, which killed about 1 million.