It is a big jigsaw puzzle and there is not just one piece missing but many pieces missing," he says.
Why do people who slaughter chickens, and would seem most at risk, seem immune? Why does the flu seem to strike mainly within families? Why have there been no foreigners among the stricken?
"When it first appeared we thought it was SARS," he said. "But it didn't follow rules of past diseases like many victims, more dead, as well as less time and space between outbreaks."
...Peter Horby, a medical epidemiologist with the WHO in Hanoi, gives Vietnamese authorities high marks for most measures they have taken.
But he warns: "We need to resolve the mystery of bird flu, not just contain it and learn to live with it."
He said the number of outbreaks dotted around Vietnam shows bird flu is still "entrenched" in various parts of the country.
Horby fears the day when 20 people come down with bird flu on the same day and in several different locations.
"But we are not there yet," he says.
Here is the story.