Analysis of samples of the H5N1 bird flu virus from two of its victims in Turkey has detected a change in one gene in one of two samples tested, but it is too early to tell whether the mutation is important, the World Health Organization said Thursday.

The mutation, which allows the virus to bind to a human cell more easily than to a bird cell, is a shift in the direction of the virus being able to infect people more easily than it does now. However, that does not mean the mutation has taken root.

"We assume this could be one small step in the virus' attempt to adapt to humans," said WHO virologist Mike Perdue. "But it's only seen in one isolate and it's difficult to make sweeping conclusions. We just have to wait and see what the rest of the viruses [from Turkey] look like."

Turkey has seen an unusually high number of cases in a short period of time. Experts are investigating why.

Health authorities there raised the number of people infected with H5N1 from 15 to 18 on Thursday, after the virus turned up in preliminary tests on two people hospitalized in southeastern Turkey and in a lung of an 11-year-old girl who died last week in the same region.

All the victims are thought to have close contact with infected poultry. Samples from several of those cases are being sent to a laboratory in Britain for analysis.

Perdue said the U.N. health agency is not alarmed by the finding in a single virus sample because this exact genetic change has been seen before, in samples from southern China in 2003, and it had no impact on the course of the disease, the behavior of the virus or the pattern of human infections.

"If we saw it in more than 50 percent of samples, it would suggest the virus is really trying to adapt to humans and it would be problematic," he said.

Even if the mutation is confirmed in more samples, that does not necessarily mean it is an important enough change on its own to make the virus easily transmissible between humans, Perdue said.

The 1918 flu pandemic, the biggest in recorded history, became a global killer only after the virus slowly made a series of genetic mutations.

Influenza viruses are notoriously volatile, and experts expect to see mutations frequently. Many mutations are meaningless, or happen in only a minority of the virus samples, but specialists are watching the H5N1 virus carefully to pick up any important changes as early as possible.

Although nothing can be done to stop the mutations, tracking them is considered the best way to anticipate the next human flu pandemic.