By Anita Manning, USA TODAYThe bird flu that spread across Europe and into Africa this year seems to be slowing as warmer weather approaches, but flu experts say that doesn't mean the virus has been controlled or that it no longer poses a threat to humans.

In January and February, the deadly strain of bird flu known as H5N1, which had been spreading in Southeast Asia since late 2003, turned up for the first time in more than a dozen countries, including India, Nigeria and countries of Western Europe.

Scientists feared that with the spring migration of wild birds, it would spread further in Africa and Europe. But in recent weeks, "we haven't seen the virus move to as many countries as we saw in the beginning of the year," says Maria Cheng, a spokeswoman for the World Health Organization. Reports of human cases also may be declining compared with earlier in the year, she says.

Cheng says WHO has been monitoring the H5N1 virus and its movement closely since early 2003, and every year "it does tend to drop off activity in the summer."

The virus has killed millions of birds and infected more than 200 people, killing more than half of them. Nearly all had close contact with infected poultry, but the human infections have raised worldwide concern that if the virus adapts in a way that allows it to spread easily from person to person, it could cause a flu pandemic.

Flu pandemics have occurred throughout history, and health experts warn that another one is inevitable, though they can't predict when and don't know what virus will be the spark.

"H5N1 does have worrying characteristics," Cheng says. "Its spread is enormous, it continues to infect humans and it has a high fatality rate. But we don't know if this will ever materialize into a pandemic virus."

Veterinarian Richard Slemons, a bird-flu expert at Ohio State University, says flu spreads more easily in dense populations of birds, but at this time of year, wild birds are nesting and not congregating in large flocks. That could mean infected birds are not being tested because they're out of sight, or that the virus is not being transmitted by wild birds, as many had expected. The role of wild birds in the spread of the H5N1 virus is unclear, he says.

"We have circumstantial evidence that wild birds can be involved in spreading the virus some distance, but it's circumstantial."

Fluctuations in flu activity are natural, says researcher Richard Webby of St. Jude Children's Research Hospital. "I wouldn't think it's all over," he says. "I just think it's a natural lull." Flu is primarily a winter virus, and "I wouldn't be surprised if as we approach the winter months again, it takes off."

The rate of new infections in people may decline as people in rural parts of the world learn of the danger of close contact with sick birds, Webby says, but "I wouldn't say we've dodged a pandemic."