It was an ideal place to set up traps.

The search for bats is part of an investigation into a deadly new viral disease that has drawn scientists from around the world to Saudi Arabia. The virus, first detected there last year, is known to have infected at least 77 people, killing 40 of them, in eight countries. The illness, called MERS, for Middle Eastern respiratory syndrome, is caused by a coronavirus, a relative of the virus that caused SARS (severe acute respiratory syndrome), which originated in China and caused an international outbreak in 2003 that infected at least 8,000 people and killed nearly 800.

As the case count climbs, critical questions about MERS remain unanswered. Scientists do not know where it came from, where the virus exists in nature, why it has appeared now, how people are being exposed to it, or whether it is becoming more contagious and could erupt into a much larger outbreak, as SARS did. The disease almost certainly originated with one or more people contracting the virus from animals — probably bats — but scientists do not know how many times that kind of spillover to humans has occurred, or how likely it is to keep happening.

There is urgency to the hunt for answers. Half the known cases have been fatal, though the real death rate is probably lower, because there almost certainly have been mild cases that have gone undetected. But the virus still worries health experts, because it can cause such severe disease and has shown an alarming ability to spread among patients in a hospital. It causes flulike symptoms that can progress to severe pneumonia.

The disease is a chilling example of what health experts call emerging infections, caused by viruses or other organisms that suddenly find their way into humans. Many of those diseases are “zoonotic,” meaning they are normally harbored by animals but somehow manage to jump species.

“As the population continues to grow, we’re bumping up against wildlife, and they happen to carry some nasty viruses we’ve never seen before,” said Peter Daszak, a disease ecologist and the president of EcoHealth Alliance, a scientific group that studies links between human health, the health of wild and domestic animals, and the environment.

Saudi Arabia has had the most patients so far (62), but cases have also originated in Jordan, Qatar and the United Arab Emirates. Travelers from the Arabian peninsula have taken the disease to Britain, France, Italy and Tunisia, and have infected a few people in those countries. Health experts are also worried about the Hajj, the Muslim pilgrimage that will draw millions of visitors to Saudi Arabia in October.

MERS has not reached the United States, but health officials have told doctors to be on the lookout for patients who get sick soon after visiting the Middle East. So far, more than 40 people in 20 states have been tested, all with negative results, according to Dr. Anne Schuchat, the director of the National Center for Immunizations and Respiratory Diseases at the Centers for Disease Control and Prevention.

The illness can be spread by coughs and sneezes, or contaminated surfaces, and people with chronic diseases seem especially vulnerable. More men than women have fallen ill, possibly because women have been protected by their veils. A cluster of cases that began in a Saudi hospital in April ultimately involved 23 people, including several family members and health workers. One man infected seven people, each of whom spread the disease to at least one other person.

Regardless of where they emerge, new illnesses are just “a plane ride away,” said Dr. Thomas Frieden, the director of the C.D.C.

And while MERS is not highly contagious like the flu, he said, “the likelihood of spread is not small.”

Ailing Patients Most Vulnerable

In May, Saudi health officials asked an international team of doctors to help investigate the hospital cluster. One concern was that a number of cases were in patients at a dialysis clinic, and doctors feared that dialysis machines or solutions might be spreading the disease.

“It was pretty easy to figure out that couldn’t have been the case,” said a member of the team, Dr. Connie S. Price, the chief of infectious diseases at Denver Health Medical Center.

The patients’ records did not point to dialysis as the culprit, she said, and there were clear cases of transmission in other parts of the hospital that had no connection to dialysis.

Why, then, the outbreak among dialysis patients? The answer seems to be that they were older, chronically ill and often diabetic; diabetes can suppress the immune system’s ability to fight off infections. So, when one dialysis patient contracted MERS, others who happened to be in the clinic at the same were easy targets for the virus.

“Introducing it into a dialysis center gives it the perfect environment to spread among vulnerable patients sitting in open bays for many hours,” Dr. Price said.

Some health experts have suggested that MERS, like SARS, may fade away. The SARS outbreak erupted in early 2003, but ended by that summer. Much of the success was attributed to infection control in hospitals and also to eliminating animals like civet cats, which were thought to have caught the virus from bats and to be infecting people in markets where the civets were being sold live to be killed and eaten.

But Dr. Allison McGeer, a microbiologist and infectious disease specialist at Mount Sinai Hospital in Toronto who is also part of the team that studied the Saudi hospital outbreak, said there were no signs that MERS was going away.

“Absolutely not,” she said. “There are ongoing cases of disease acquired in the community. The first we know about is April 2012 in Jordan. There has been a steady and continuing number of cases.”

The fact that the disease has apparently emerged in geographically disparate places, with widely scattered cases in four Middle Eastern countries, also makes Dr. McGeer doubt that it is simply going to fizzle out.