Researchers say a Thai girl who died of bird flu in September last year probably infected her mother and aunt, in what would be the first documented case of human-to-human transmission of the deadly virus.

The scientists say they analyzed virus samples obtained from the three patients and created a timeline of possible exposure to the virus and development of symptoms.

They concluded that the girl was the source of infection for both women. However, the researchers say they found no evidence that the bird flu virus has changed in any way to make human-to-human transmission easier in the future.

The study will appear in the New England Journal of Medicine later this week.

Some information for this report provided by Reuters.