A man in Saudi Arabia wore a mask last week to protect against a virus that has killed 22 since it was found there last year. Credit Reuters

A new flu, H7N9, has killed 36 people since it was first found in China two months ago. A new virus from the SARS family has killed 22 people since it was found on the Arabian Peninsula last summer.

In past years, this might have been occasion for panic. Yet chicken and pork sales have not plummeted, as they did during flus linked to swine and birds. Travel to Shanghai or Mecca has not been curtailed, nor have there been alarmist calls to close national borders.

Is this relatively calm response in order? Or does the simultaneous emergence of two new diseases suggest something more dire?

Actually, experts say, the answer to both questions may well be yes.

“We’ve done a great job globally in the last 10 years,” said Dr. William B. Karesh, a wildlife veterinarian and chief of health policy for the EcoHealth Alliance, which tracks animal-human outbreaks. “Compared to H5N1 and SARS, we’re getting on top of these diseases much, much faster.”

But he added that “people have become desensitized over time — it’s ‘Oh, O.K., another one.’ ”

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And scientists say the world cannot afford to relax. The threat is real. New diseases are emerging faster than ever.

Peter Daszak, a parasitologist and president of the EcoHealth Alliance, has even put a number on it: 5.3 new ones each year, based on a study using data from 1940 to 2004. He and his co-authors blamed population growth, deforestation, antibiotic overuse, factory farming, live animal markets, bush meat hunting, jet travel and other factors.

Some aspects of the new viruses are scary. The Arabian coronavirus — now officially named MERS, for Middle East respiratory syndrome — has killed about half of those it infects, while SARS killed less than a quarter; in the lab, it replicates faster than SARS, penetrates lung cells more readily and inhibits the formation of proteins that warn the body that it is under attack.

In her closing remarks on Monday at the annual meeting of the world’s health ministers, Dr. Margaret Chan, director-general of the World Health Organization, said the virus was now her “greatest concern.”

Until experts figure out where it hides and how it infects humans, “we are empty-handed when it comes to prevention,” she said. “These are alarm bells, and we must respond.”

The H7N9 flu has been fatal in a quarter of known cases — the 1918 Spanish flu killed only 2 percent of its victims — and already has one dangerous mutation that helps it replicate at human body temperatures.

Still, better surveillance means that such threats are being caught sooner, giving time to develop countermeasures like vaccines and making it far less likely that a virus like the 1918 flu will ever again kill millions.

It also means that outbreaks that once might have faded away unnoticed now set off alarms, for better and for worse. Fifty years ago, even the dreaded H5N1 bird flu, which emerged in 2003 and kills about half its victims, might have been missed. It makes the jump to humans so rarely that even now it is basically a poultry problem: It has killed millions of chickens and occasional flocks of wild birds, but in a whole decade has claimed only 364 human lives, and that is known only because it can be distinguished from other flus by genetic typing.

The world’s ability to detect new diseases has sped up for reasons both technical and political.

First, rapid gene sequencing is now done in many laboratories.

An image of a coronavirus, part of a family that causes the common cold and SARS. A new strain, called MERS, for Middle East respiratory syndrome, has been particularly lethal, killing half those infected. Credit Uncredited/Health Protection Agency, via Associated Press

Second, accurate symptom descriptions are instantly available. Web-based news services like ProMED, with scientist-members all over the world, issue several daily reports of outbreaks of everything from banana wilt to sheep bluetongue to human Ebola. Also, genetic sequences of new viruses are often posted on public databases, so their travels can be tracked. Scientists learned, for example, that a 2008 convention of Roman Catholic youth in Sydney, Australia, drew in influenza strains that then seeded new outbreaks all over the Northern Hemisphere.

Third, and very important, countries that used to hide their outbreaks now admit them. It would be virtually impossible now, for example, to repeat what happened in Africa in the 1980s, when presidents insisted for years that no one in their countries had AIDS.

The paragon of the new transparency cited most often is China. In 2003, it was excoriated for covering up its SARS outbreak. It later dismissed many of the officials involved. Now, with H7N9, “they’re being forthright and they’re also right at the forefront of research,” said Dr. W. Ian Lipkin, a microbe hunter at the Mailman School of Public Health at Columbia University, who just opened a partner laboratory at China’s Centers for Disease Control.

Saudi Arabia suffered a similar embarrassment in 2005, when it reacted slowly to polio spreading toward Mecca with pilgrims from northern Nigeria. Cases of paralysis ultimately reached the hills outside Mecca and from there spread briefly as far as Indonesia. Saudi Arabia now gives polio vaccines to millions of pilgrims on arrival.

Covering up an outbreak is now a violation of World Health Organization regulations adopted in the wake of SARS. The rules require members to disclose any public health event that could spread beyond their borders.

Both H7N9 and MERS fit that description. Neither is easily transmissible, though both have almost undoubtedly infected family members, nurses or hospital roommates after long exposure. Most deaths from both have been in older patients with other health problems.

More worrisome is that no one knows how these viruses first infect victims.

H7N9 is avian, a mix of genes from domestic chickens and wild waterfowl. But many Chinese H7N9 patients have had no known bird contact, and the disease has been found only rarely in birds. Unlike H5N1, it does not wipe out flocks, so it is hard to hunt. Its spread pattern is roughly circular around Shanghai, suggesting it is mostly in poultry, not migratory birds. That could change if it starts traveling in wild ducks. (Rice farmers have duck farmers drive flocks into paddies to eat the snails that eat rice shoots, and wild ducks mix with them there.)

A decade ago, H5N1 also started in China but spread west in a zigzag pattern as wild waterfowl shared Mongolian lakes in summer with species that went southwest to Eastern Europe, Egypt and Africa and were caught in storms that blew them as far as Britain.

The origins of MERS are even more baffling. Scientists assume it is from bats, because it is genetically closer to coronaviruses found in them than to SARS or to the four known human coronaviruses, which cause common colds. But while bats in Mexico, Europe and Africa have similar viruses, none have yet been found in Arabian bats or in camels, goats or other animals that might transfer it to humans.

Dr. Daszak cited Nipah virus as an example of how humans get bat diseases. It was the inspiration for the 2011 movie “Contagion,” in which Gwyneth Paltrow had vivid death and autopsy scenes. Bat feces landed on fruit eaten by pigs, and Ms. Paltrow’s character was infected when she shook the unwashed hand of a casino chef who had just cleaned out a dead pig’s mouth. (In the first real-life Nipah outbreak, in Malaysia in 1999, most victims were pig farmers and butchers.)

But another study, done in Bangladesh by a colleague of Dr. Daszak, showed that humans get Nipah directly from bats by drinking fresh date palm sap. Sap-drinking bats crawled into the collecting jugs hung in trees, drooling and urinating in them.

Small numbers of sap drinkers may have died of Nipah for decades without it being noticed, Dr. Daszak said.

Right now, doctors are relying on isolating patients and antiviral treatment with oseltamivir and zanamivir for H7N9, and ribavirin and interferon for MERS.

If either virus goes epidemic, the next step would be vaccine.

The Centers for Disease Control and Prevention began making one against H7N9 in early April. The first of several candidates may be ready for manufacturers by the end of May, a spokeswoman said. How long it then would take to make and package millions of doses is unpredictable, she said, but should take at least six additional months.

Any vaccine for MERS will take much longer, said Mark A. Pallansch, director of the C.D.C.’s viral disease division. While flu vaccines have been produced around the world for 60 years, the passion for a coronavirus vaccine has faded since the SARS epidemic. Until recently, the most interested parties were poultry farmers, since one coronavirus kills turkeys.

Coronaviruses are unusually complex, so finding potential vaccine targets has been hard, and the extensive safety testing is expensive. Also, an animal model for testing was only recently found — macaque monkeys, in which the virus causes pneumonia.

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