Experts are calling for urgent action to set up global vaccine factories to deal with a possible pandemic of bird flu.
The infection is spread by birds
So far, only small batches of vaccine against the H5N1 strain of bird, or avian, flu have been produced.
But Dr Klaus Stohr of the World Health Organization says this is nowhere near enough.
The warnings follow reports this week of a possible case of human-to-human transmission of the virus.
Prior to that, all other confirmed cases of the infection in humans had been caught from birds.
Altogether, Thailand has reported 15 cases of avian flu in humans, of which 10 have proved fatal, since January of this year.
Dr Stohr, head of the WHO's global influenza programme, said this was the possible beginning of global spread of the virus.
"The situation in Asia is very concerning," he told BBC Radio 4's Today programme.
"We have an avian influenza virus which is very widespread but which has not yet, fortunately, transmitted easily, rapidly and for a long period of time between humans.
"But the capacity is there and we have a chance now to prepare for it.
"It would be foolish to sit and lean back and wait for it to happen."
Scale up production
He urged countries to act now to develop vaccines against the virus.
"It took two months for the WHO to develop the prototype strain to give it to companies.
"And now five months have already passed and only two companies have invested into the development of small clinical trial ventures.
"We are still months down the road before any large scale production could start."
He said about 300 million doses of the vaccine could be produced globally using current facilities.
If a pandemic were to occur he said 2.5-3 billion doses would be needed.
Earlier this week, the UN Food and Agriculture Organization (FAO) and the World Organization for Animal Health (OIE) said bird flu was a permanent threat to animal and human health that continued to exist.
They too urged governments to take more action.