HANOI, Vietnam - Bird flu has resurfaced with a vengeance in Vietnam — with five people falling ill in as many weeks — after no human cases had been reported for a year and a half.

Health experts say the spike is a sobering reminder that the H5N1 virus remains deep-rooted and can kill at any time. The virus also has flared elsewhere, with people falling ill in China, Egypt and Indonesia this month alone. And poultry outbreaks have surfaced in Myanmar, Malaysia and as far afield as the Czech Republic.

Vietnam, previously hailed as Asia"e;s bright spot for beating back the virus, has seen an unexpected surge since last month, when it reported its first human case since November 2005. Two patients have died, two have recovered and one is critically ill.

"It"e;s always been lingering and loitering, but now it"e;s striking and we don"e;t know why," said Peter Cordingley, spokesman for the World Health Organization"e;s Western Pacific region. "I think the first lead that we might follow is, have people begun to drop their guard?"

Vietnam was blindsided when the virus began ravaging poultry stocks across Asia in late 2003. The country logged dozens of human deaths and suffered huge financial losses before undertaking an ambitious campaign to vaccinate all poultry.

The plan worked well, and no outbreaks were reported throughout 2006 until the virus re-emerged earlier this year among birds. The latest flareup began in May and has affected poultry in 18 provinces, killing or forcing the slaughter of some 200,000 birds.

Four of the human cases were from the north and one was from central Vietnam, raising the bird flu death toll in the country to 44.

"The virus has all the time had the capability" to infect humans, said Hans Troedsson, WHO representative in Vietnam. "Why it happened now in May and didn"e;t happen in January and February, we don"e;t know."

Agriculture officials say unvaccinated ducks are largely to blame for the recent problems. In March, the government lifted a ban on hatching and restocking waterfowl, which has led to more ducklings being raised and transported without being immunized. Vaccination helps to decrease the spread of the virus, but even that is not foolproof because ducks must receive multiple shots each year to ensure immunity.

"It"e;s ducks and it"e;s the duck movement and the upsurge of ducks on the rice farms — all these things are really the major cause of this wave," said Andrew Speedy, the U.N. Food and Agriculture Organization representative in Vietnam.

He said the government remains committed to fighting bird flu and has bought 200 million more doses of vaccine. The prime minister also has issued new orders for officials to increase their vigilance in a country where many people now show little fear of bird flu.

"People keep thinking it is impossible for the virus to come back to their villages," said Hoang Van Nam, deputy director of the Animal Health Department. "I think this is partly because the last time we managed to control it, so people have been too confident."

The virus has killed at least 191 people worldwide, according to the WHO. Vietnam"e;s latest cases have not been added to the health agency"e;s tally because they have not been confirmed by an outside laboratory. The virus remains hard for humans to catch, but experts fear it could mutate into a form that spreads easily among people, potentially sparking a global pandemic. So far, most human cases have been traced to contact with infected birds.