Wang Zhao / AFP - Getty Images

A new strain of bird flu identified in China "is one of the most lethal influenza viruses we have seen so far," Dr. Keiji Fukuda, the World Health Organization (WHO)'s Assistant Director-General for Health Security, tells journalists at a press conference in Beijing on Wednesday.

BEIJING – A new type of bird flu that has killed 22 people in China since March is one of the most deadly strains of influenza known, international health experts said on Wednesday. 

"This is one of the most lethal influenza viruses we have seen so far," said Dr. Keiji Fukuda, the World Health Organization (WHO)’s Assistant Director-General for Health Security. "We are at the beginning of our understanding of this virus."

The H7N9 strain appears to spread more easily to humans than SARS, a different virus that started killing people in Asia a decade ago, experts said. Severe acute respiratory syndrome killed around 800 people globally in 2003 before it was stopped.

"This is an unusually dangerous virus for humans," added Fukuda, who was speaking in Beijing alongside leading flu experts from around the world.  

The delegation from United States, Europe, Hong Kong and Australia, as well as China, have just concluded a week-long investigation that took them to affected areas in Shanghai and Beijing.

Little is known
The group of experts made an impressive display of international cooperation, but at the same time admitted just how little is known about the virus that has infected 108 people since March.

"We are at the very early stages of this investigation," said Dr. Nancy Cox, who heads Influenza Division at the Center for Disease Control and Prevention in Atlanta. "There's a lot to be learned.”

Most of the cases so far have been found in eastern China, around the Yangtze River delta, but in recent days there have been cases in central and northern China, including the capital. Most have been what Fukuda called "sporadic cases."  

He said a few family clusters have been found, which could be the result of exposure to the same source of virus, or limited person-to-person transmission.

But he said: "'Evidence so far is not sufficient to conclude there is person-to-person transmission. Moreover, no sustained person-to-person transmission has been found.”

The experts concluded that live poultry markets were the most likely source of infection.

The experts praised the swift action of Chinese authorities in closing live poultry markets, and said it was "encouraging" that there have been no new cases in Shanghai since its markets were shuttered.

And they called for continued international cooperation against a virus that doesn't recognize borders. 

"The risks of an outbreak situation are shared in a globalized world, where we are all interconnected," said Fukuda.

Legacy of distrust
All of those who spoke today went out of their way to praise the response and of the Chinese authorities and their openness and transparency. There is enormous sensitivity to any suggestion that their presence in China implies any criticism of local efforts.

China still lives in the shadow of the SARS pandemic, which began here a decade ago and killed hundreds worldwide, including in the U.S. It was made worse by an initial cover-up by the Chinese authorities.

"The response reflects earlier and strong investments in health and preparedness made by China," said Fukuda.

SARS also left a legacy of distrust, which was on display earlier in the week in Shanghai, when a press conference by the local government and WHO was gatecrashed by the daughter of a couple infected with H7N9. The 26-year-old demanded information about her quarantined father; her mother had died.

"The hospitals and medical staff appear friendly to members of the media like you but have responded in a lukewarm manner to inquiries from family members like me," she told the South China Morning Post. She was taken away by officials.

The experts said that in the absence of so much basic information about the extent of the public health risk it was critical to maintain a high level of awareness. They also noted that the weather is warming up in China, which might provide a bit of a respite and buy them some important time, since H7N9 -- in common with other influenza -- spreads less easily in the spring and summer.

Related:

• A new openness as new bird flu virus spreads in China
• Six more diagnosed with new bird flu in China
• Scientists ready to re-start bird flu experiments

This story was originally published on Wed Apr 24, 2013 6:19 AM EDT

MAE SOT, Thailand -- Clipboard in hand, Dr Francois Nosten worked his way down a ward of malaria patients. He stopped in front of five-year-old Ayemyint Than, who sat to attention and smiled. The smile told Nosten as much as his lines of graphs and figures.

Ian Williams / NBC News

Ayemyint Than, 5, is being treated for malaria in northwestern Thailand.

"She's doing well," he said, moving to an older man, whose pale face and dull sunken eyes told a very different story. "Day five, and he's still positive?" he asked another of the doctors. "That's not very good. It means he was very slow to clear the parasite, no?"

To Nosten, it was further evidence of an alarming rise in resistance to artemisinin, currently the front-line drug in the treatment of malaria. He fears it could be the start of a global "nightmare" in which millions of people could lose their lives.

"We have to beat this resistance, win this race and eliminate the parasite before it’s too late. That's our challenge now," he said.

He said that artemisinin should take about 24 hours to deal with the parasite, but it was now taking three or four days in some cases. "We are going to see patients that don't respond to the treatment anymore,” he warned.

Nosten runs the Shoklo Malaria Research Unit, which is part of the Faculty of Tropical Medicine at Thailand's Mahidol University.

The unit has a string of clinics on both sides of the Moi River, which marks the porous border between Thailand and Myanmar.

Ian Williams / NBC News

Migrants cross the Moi River, marking the border between Myanmar and Thailand.

Nosten set up the first one in 1986, since when there has been a steady fall in the total number of cases of malaria, but most recently a worrying emergence of drug resistance.

He first sounded the alarm in research published earlier this year, following the emergence of similar drug resistance along the Thai-Cambodia border.

Full health coverage from NBC News

Nosten’s not sure whether the resistance he's found has spread from the Cambodia border or is home-grown. Either way, he's worried.

"It means that all the progress of the last 10 to 15 years will be lost," he warned. "Now the resistance is here, we worry that we are running out of time."

Ian Williams / NBC News

Staff examine a baby who has been brought to the clinic with a fever, suspected to be malaria.

The malaria parasite -- carried by infected mosquitoes from person to person -- still kills an estimated 655,000 people a year.

That's almost 2,000 a day, mostly in Africa, with children being most at risk.

If the world loses its front-line drug, the impact could be devastating.

"The nightmare scenario is that the resistance will travel," Nosten said.

"We know what will happen in Africa when resistance is bad because we've been there before in the 1990s with chloroquine (another anti-malarial drug) … millions of deaths," he warned.

"We must prevent artemisinin resistance reaching Africa, but we also need to control it for the people in Asia - for their future."

Ian Williams / NBC News

Dr Francois Nosten, right, consults staff as he meets malaria patients at a clinic near Mae Sot, Thailand.

Resistance to just about every major anti-malarial drug has started in the border regions that have been home to Nosten for more than 25 years.

Nobody knows exactly why, but poverty, conflict and large migrant and refugee populations constantly on the move all likely play a part. As do fake drugs or a failure to properly complete a course of treatment.

In the case of chloroquine, once the anti-malarial drug of choice, it took less than 20 years for resistance to spread from the borders of Thailand to Africa.

Study: Mosquitoes change habits to avoid anti-malaria nets

Nosten is worried that artemisinin resistance is growing much faster than he'd anticipated, with the drug failing initially to fully clear the parasite in more than half the cases he now sees.

"It initially goes after a few days, then it comes back. We see that more and more now," he said.

"In 2009, we still had 90 percent of patients cured. In 2010, it dropped to 60 to 70 percent. Now it's about 50 percent," he added.

Ian Williams / NBC News

Migrants from Myanmar wait to be examined at a clinic on the Thai side of the border.

Some scientists claim this is too alarmist, since the parasite does eventually die, with longer treatment and higher drug doses, but Nosten sees no room for complacency.

"We have to respond quickly, not next year or three years' time. It's now or probably it will be too late," he said.

Artemisinin comes from a Chinese plant and is quick, potent and with no side effects. Little wonder it has been hailed as a wonder drug, the golden bullet in the global fight against malaria.

What makes the resistance so worrying is that there is no new drug ready to replace it.

Nosten said that although several drugs are in development, they could be five to 10 years away from deployment "if they make it  … and we haven't got five to 10 years.”

The Shoklo Malaria Research Unit runs its own labs fashioned out of a sprawling old Thai house in the border town of Mae Sot, where teams of research scientists are working to better understand the parasite and the mosquitoes that carry it from person to person.

It is here that Chiara Andolina keeps mosquitoes that are literally hand-reared -- fed from her arm, which she extends through a mesh hole into a container of the hungry creatures every three days.

"Usually I feed around 600 of them in a cage like this," she said.

Of course these are not infected mosquitoes, though watching them settle on her arm for a good lunch is not a sight for the squeamish.

Read more international coverage from NBC News

In another room, Nosten settled over photographs showing the rapid development of the parasite once it has invaded a blood cell.

"If you can kill them very, very young -- like these -- they don't have time to develop into big fatty ones," he said, his pen jabbing at the photo. "These fatty ones are the ones that get stuck in your brain and kill you."

In other rooms, the DNA of parasites was being isolated and sequenced and drugs were being tested as part of Nosten and his team’s efforts to figure out what's behind the emerging resistance.

They are also looking for vulnerabilities and new ways to attack their enemy.

"It's hugely important to understand what's going on and contain it if we can," Nosten said. "We need to try things. We need to explore. It’s like exploring new territories in malaria."

Bazell: Malaria vaccine a half-effective, temporary protection

The French scientist has spent most of his working life in the tropics, initially with the medical humanitarian group Médecins Sans Frontières.

He believes he is engaged in a vital battle -- "a race against malaria" -- as he puts it.

After so many years on the malarial front lines, the battle has become deeply personal.

He dreams of completely eliminating this familiar but wily enemy.

However, he also knows that with the emergence of artemisinin resistance the stakes have never been higher.

More world stories from NBC News:

• Drug-resistant malaria threatens deadly global 'nightmare'
• From alcohol to kites: An A to Z guide to the Islamic Republic of 'Banistan'
• UK police: Attackers dressed as Oompa Loompas beat man
• Vatican launches swipe-card security system
• US sailors sue Japan's TEPCO for post-quake radiation exposure

Follow World News from NBCNews.com on Twitter and Facebook