As the scientists peered into the darkness, their headlamps revealed an eerie sight. Hundreds of eyes glinted back at them from the walls and ceiling. They had discovered, in a crumbling, long-abandoned village half-buried in sand near a remote town in southwestern Saudi Arabia, a roosting spot for bats.

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.