The woman, identified only as Pranom, was confirmed to have the H5N1 virus but her condition was improving, Mr Charal Trinwuthipong, director general of the Disease Control Department, told reporters. 

Nine people have died of bird flu in Thailand, including an 18-year-old man last month, in the latest round of the disease that began in July. Eight other people died earlier in the year.

The disease also has killed 19 people in Vietnam, and tens of millions of chickens and other birds have died or been culled throughout much of Asia.

A test on Ms Pranom on Friday had turned out negative for bird flu but a second test gave a positive result, Mr Charal said.

Although she is believed to have come in contact with chickens, authorities are still investigating the possibility that she got the disease through human-to-human transmission.

´This is a theory. We are looking at the possibility of the virus spreading from human to human but no one can conclude yet,´ he said.

Ms Pranom´s sister, Pranee, 26, died on September 20 in a hospital near Bangkok. She is not known to have come in contact with chickens or birds, but had spent about a week in close contact with her 11-year-old daughter until the girl´s death in a hospital on September 12 in the northern Kamphaengphet province.

The girl also had bird flu-like symptoms, but no tests could be done on her because she had already been cremated by the time officials were alerted.

Health experts are worried about the possibility of the avian virus combining with a human influenza virus to create a more deadly version that could easily be spread among people - giving rise to a possible global pandemic.

A bird flu outbreak in Hong Kong in 1997 killed six people and resulted in limited human-to-human transmission. Those who got the disease from other people experienced only minor symptoms, and there was no evidence the virus had mutated into a highly dangerous form.