A Thai woman who died of bird flu possibly caught it from her daughter, but there was no indication the virus would spread more widely among humans, a top World Health Organization (WHO) official said on Tuesday.

Although the H5N1 virus is primarily transmitted by birds, there have been isolated cases of human-to-human transmission in the past, notably in Hong Kong in 1997.

“What we’re seeing in Thailand could possibly…be again one of these…non-sustained inefficient, dead-end-street human-to-human transmissions,” said Klaus Stohr, head of the WHO’s global influenza program.

Nevertheless, he said that the case was of concern because of the possibility that the virus might have mutated to become more easily transmittable, even if there is no evidence of this yet.

“We are concerned, however, because the occurrence of such a cluster (of cases) could also indicate the beginning of a more widespread transmission…which could lead to the global spread,” he told a news conference.