HANOI — Tran Minh Giang has spent more than a third of his young life in a Vietnamese hospital, and it could be many months more before he can go home. His ailment stems from a disease that in Asia is as common as chicken pox and usually about as severe.

The 20-month-old boy was sickened by a particularly menacing form of hand, foot, and mouth disease that has killed hundreds of young children in the region. They sometimes suffer high fever, brain swelling, paralysis, and respiratory shutdown, even though they may have been infected by people with few or no symptoms.

When the strain hit Cambodia recently, doctors had no idea what it was, and even now researchers don’t fully understand why it can be so devastating. Seven months after becoming sick, Giang still breathes using a ventilator connected to a hole in his throat.

‘‘It may take time, maybe years, before he can recover. When he sleeps, his lungs don’t work,’’ his father, Tran Nam Trung, said Thursday while fanning the toddler. ‘‘When he first got a high fever, I didn’t think that he would be in a situation like this.’’

The enterovirus 71 strain, or EV-71, raised fears earlier this week when it was detected in some lab samples taken after 52 of 59 Cambodian children died suddenly from a mystery illness that sparked international alarm. The World Health Organization said via Twitter on Thursday that its investigation has found most of the cases were caused by the disease.

A specialist at the UN health agency earlier said this was the first time EV-71 had been identified in Cambodia, but it’s a well-known pathogen elsewhere in Asia. In the first half of this year alone, 356 people in China and 33 in Vietnam have died from it.

The scale of the disease was clear last week on the crowded ground floor of a hospital in China’s hard-hit central Hunan Province. Dozens of crying children were packed into two small rooms, sitting or lying on chairs with IV drips hooked to them. Hunan reported 33 hand, foot, and mouth disease deaths in May, a quarter of the country’s total that month.

The disease has exploded across the region since 1997, when the first major outbreak was reported in Malaysia. Since then, Japan, South Korea, Singapore, Mongolia, Taiwan, and Australia have all wrangled with it.

EV-71 is one of a group of viruses that cause the disease, but it has become a more dominant strain over the past decade in Asia. Still, only a small percentage of children infected experience severe symptoms, and researchers aren’t sure why. There is no vaccine or specific treatment to cure it.

‘‘There’s a buildup of that susceptible population, like many viruses, and this happens to be the children who have not been exposed to different types of enteroviruses before,’’ said Dr. Zarifah Hussain Reed, coauthor of a WHO report on hand, foot, and mouth disease and medical director at a Malaysia-based biotech company researching a vaccine for it. ‘‘Then this buildup somehow explodes in a way that suddenly you get severe cases of EV-71.’’

She said it is unclear why the virus remains largely confined to certain parts of Asia — India and Indonesia, for instance, have not reported large outbreaks — and it’s not understood if EV-71 is more infectious or perhaps just better at invading the neurological system than other strains.

The disease is in the same family as polio and gets its name from the telltale symptoms it causes, including rash, mouth sores, and blisters covering the hands and feet. It mainly affects children younger than 5, and is difficult to control because it spreads easily through sneezing, coughing, and contact with fluid from sores or infected feces.

In day-care centers and schools, it’s nearly impossible to keep little blistered hands from coming into contact with other children and everything they touch. Another problem is that many infected children never get sick, but they can still transmit the virus to others. Frequent hand-washing and disinfection of toys and surfaces are advised, and sometimes schools are forced to close to help halt the spread.

The first EV-71 infection with neurological symptoms was reported in California in 1969. Outbreaks have since occurred periodically in the United States and Europe, but the disease has been a stubborn menace in Asia, typically occurring in cycles. Some doctors have warned that if the virus isn’t controlled, it can jump borders and threaten other regions as well.

Dr. Pham Nhat An, vice director of the National Hospital of Pediatrics in Hanoi, says he has dealt with the disease for three decades. But it didn’t start killing until two years ago, when the number of hospitalized cases started to spike.

‘‘It’s worrying,’’ he said in his office. ‘‘We need to think about a vaccine.”

In a unit on the other side of the building, Trung sat on the edge of a bed fanning little Giang, who probably caught the disease from a mildly ill cousin.

Giang was just 13 months old when he came down with a fever. He didn’t seem overly sick and continued to play, so his parents believed it was a bug that would pass. By morning, the baby was purple and convulsing. His lungs were shutting down.

Now the chubby boy can sit, and he occasionally musters a quick smile. But he remains lethargic, with tubes running out of his nose and throat. Trung had to quit his job to help his wife care for the child 24 hours a day in the hospital, which is common in many parts of Asia where nursing staff is thin.

‘‘People need to be very cautious and they need to strengthen surveillance to try to prevent the disease,’’ Trung said.