The boy, who died in hospital on Wednesday, was not known to have had direct contact with chickens, health officials said.
On Friday China also reported a new case of H5N1, the fifth person in the country known to have been infected with the deadly virus.
The 31-year-old woman, who lived in Heishan county of Liaoning province, has since recovered.
The World Health Organisation (WHO) said it may seek to investigate control measures in the area, which has not reported any other human cases.
The death of the five-year-old boy from the central province of Nakhon Nayok, 110km from Bangkok, took Thailand's bird flu death toll to 14 out of 22 known cases since the virus swept through large parts of Asia in late 2003.
Source of flu
It was not certain how the boy caught the virus, which usually strikes those in close contact with infected fowl or their droppings, senior health officials said.
"We believe that the boy contracted the virus from his surroundings because, although his family does not raise chickens, there are chickens raised in his neighbourhood," said Thawat Suntrajarn, head of the Health Ministry's Disease Control Department.
That would follow the usual pattern of human infections of the virus, which has not yet shown signs of evolving into a form which could pass easily from person to person.
The H5N1 virus is now endemic in
poultry in SE Asia and beyond
Experts say that is the great fear. If the H5N1 virus did acquire that ability, it could set off a pandemic which could kill millions of people without immunity to the new strain.
The virus is now endemic in poultry in parts of Asia and countries around the world are preparing plans to deal with a pandemic which could cause massive economic losses as well as millions of deaths.
A pandemic could cause a serious recession in the US economy, with immediate costs of between $500 and $675 billion, according to two new reports.
New Jersey based WBB Securities LLC predicted a pandemic could cause a one-year economic loss of $488 billion and a permanent economic loss of $1.4 trillion to the US economy.
The World Bank has predicted a pandemic could cost the global economy $800 billion a year.
If the virus mutates into a form which passes between humans, it is likely to closely resemble the 1918 pandemic strain of flu that killed anywhere between 20 million and 100 million people, separate reports released by WBB and the Congressional Budget Office (CBO) said.
"... those who survived would miss three weeks of work, either because they were sick, ecause they feared the risk of infection at work, or because they needed to care of family or friends"
US Congressional Budget Office report
This means 30% of the population would be infected and more than 2% would die, the report from the CBO said.
"Further, CBO assumed that those who survived would miss three weeks of work, either because they were sick, ecause they feared the risk of infection at work, or because they needed to care of family or friends," the report reads.
The CBO said a pandemic could deal a $675 billion hit to the US economy.
Meanwhile, in a boost for countries seeking the antiviral drug Tamiflu, one of four drugs known to work against influenza, Swiss manufacturer Roche reached agreements with two US generic drugmakers, as well as 13 other drug producers, US Senator Charles Schumer said.
The agreements are meant to allow more production of the drug, known generically as oseltamivir, in case of an avian flu pandemic, Schumer, a New York Democrat, said in a statement.
Roche has struck a deal to allow
greater US production of Tamiflu
Countries are seeking to stockpile the drug but all are many million doses short of what would be needed to treat a pandemic.
Last month, Roche said Indonesia, Thailand and the Philippines were free to begin making their own versions of the pill because it does not have patent protection in those countries.
Doctors believe Tamiflu may help control a pandemic of H5N1 influenza, although evidence suggests it may be less effective than it is against seasonal flu.