Geneva ? For years after its appearance in 1997 in Hong Kong, the current bird flu virus seemed corralled in a few east Asian countries. But in the past four months, it has spread across Europe and into Africa, bringing to 31 the number of countries with sick birds.

People have caught it in a quarter of those, and just six people outside east Asia have died. The virus is still not easily caught by humans.

Even so, its sudden sweep across continents on the wings of birds has stunned public health officials. And most say they cannot predict where or when this disturbing germ might mutate into a form that could unleash a deadly flu epidemic.

“Anywhere the virus lands,” said Dr. Mike Perdue, of the global influenza program for the World Health Organization.

For many months, most experts said Asia was the most likely starting point because of its large population and ubiquitous animal markets. And many still believe that. But it’s all speculation.

“It could be Asia. It could be somewhere else,” said Maria Cheng, a spokeswoman for the World Health Organization.

“Turkey would have been logically a place that you wouldn’t want to see it happen because we saw many cases in a short period of time,” said Perdue, referring to the frightening spurt of human cases and the deaths of four children in January.

Dr. Scott Dowell, of the U.S. Centers for Disease Control and Prevention, said it’s difficult to predict the spread of the disease in the next few months, because public health officials have little experience tracking a disease spread by migratory birds.

“I’m not an expert on migratory birds carrying human pathogens. I don’t know who is. We’re going beyond our experience,” said Dowell who leads CDC’s global disease detection program.

As to where the virus has the best chance of morphing into a more serious human threat, Nigeria, where many people live close to their chickens, poses a particular threat. Its 800 federal veterinarians lack the support they need to combat the disease, said Dr. Peter Cowen, former director of a WHO coordinating center in veterinary public health.