Hanoi, April 20. (AP): Bird flu remains extremely hard for people to catch, but it strikes with a deadly force in the unlucky few who become infected, new research suggests.

It's a good-news, bad-news message that has frustrated scientists trying to understand the disease and head off a possible pandemic.

In one of the largest studies of its kind, scientists in Cambodia took blood samples from 351 people in a small village, where one of the country's six bird flu deaths traced to the H5N1 virus was confirmed.

They found no antibodies for H5N1 in any of the specimens, indicating nobody became infected but recovered after falling only mildly ill or displaying no symptoms whatsoever.

The results mirror similar studies conducted in Vietnam, Thailand and Indonesia, where the virus has shown up in birds and the people it has infected, but left few other clues.

While experts stress more research is needed to determine the disease's true virulence, the research strengthens the World Health Organization's grim assessment that H5N1 kills about 56 per cent of those infected.

The good news, doctors say, is that most people don't catch the virus in its current form. The bad news is that those who do are in grave danger.