The illness begins with flu-like symptoms that can then progress to a lung infection or even to kidney failure.

"The mortality rate is at around 30 or 40 percent," says Gregory Härtl, a spokesman for the World Health Organization (WHO). "As long as we don't know how the virus behaves and how we can control it, the virus remains a danger."

The MERS virus (Middle East Respiratory Syndrome coronavirus) is one of a group of so-called coronaviruses that also includes the SARS virus and which emerged around a decade ago. In the meantime, SARS has vanished. Other viruses included in the group tend to cause just light respiratory ailments in people.

That's not true when it comes to the MERS virus, which was identified two years ago, says Udo Buchholz of the Robert Koch Institute.

"It was identified because it led to unusually severe illnesses. So people began to try and find out what the cause was. Test panels were developed in order to recognize the virus in future cases," he told DW.

Treatment?

There is no therapy specifically tailored to the virus; there are no medications and no vaccines. Only the symptoms can be treated.

Experts agree that the best means of prevention is good hygiene, since the virus is spread in an airborne fashion, such as when an infected person sneezes or coughs. The virus can also survive for some time on surfaces.

One way the virus can spread, Hartl says, is "if the infection prevention measures aren't especially good, then perhaps you'll touch a surface, and then your face."

Two components are important when it comes to estimating the extent to which the virus is dangerous, says Udo Buchholz: "How often does it emerge? How often does it lead to severe illness?"

The health expert adds that, while neither question can be answered at the moment, it could also be the case that patients with particularly severe manifestations of the illness are just the tip of the iceberg.

The source?

Although researchers are not yet certain, they believe dromedaries - also called the Arabian camel or Indian camel - are the most likely source of the infection. Many of these animals have been found to carry antibodies against MERS-CoV as well as the virus itself. Although the tests on dromedaries tended to turn out positive, they did not show any symptoms of illness. The reason for that is just one of many questions without a clear answer.

It's also unclear whether the virus spreads solely from the animals themselves to people or whether certain animal products could also be responsible for transmission. Authorities in Saudi Arabia issued a recommendation in late April to avoid eating camel meat and drinking camel milk.

Saudi Arabia is thus far home to the most instances of MERS. Cases have also been identified in Jordan, Qatar, the United Arab Emirates, Oman and Kuwait. The most strongly affected include people who have close contact with dromedaries, Buchholz says.

"You have to recognize that, in these countries, there are often farmers who keep a small number of these animals in a relatively small area - and that they have a kind of cult status there," Buchholz says, adding that the animals are often used in popular races.

Person-to-person transmission?

Meanwhile, the virus has turned up among more and more medical professionals, including doctors and nurses. The WHO reports that 60 out of 100 secondary cases - transmitted from person to person - occur among medical workers or among family members of those infected.

In Europe, there have been a few cases, as well. Buchholz says they involve individuals who were infected while in the Arab world before coming to Europe for various reasons.

"In Tunisia and in England, there've been cases where a person with a primary infection transmitted the virus to other people. But it stopped after that," says Buchholz.

When people come together in a crowded space, the danger of infection increases. "There were concerns last year that the Hajj pilgrimage would lead to a massive outbreak. That, as we all know now, is not what happened."

Various studies tested and examined large groups of those who returned from the pilgrimage, Buchholz says, with all of the results coming up negative.

Nor is there reason to panic at the moment, he says, "This relatively strong increase in infections in recent weeks is of course somewhat disconcerting, but even here, we are not seeing any longer chains of infection."

For epidemiologists, it's a key factor whether one infected person can transmit the illness to another, who can then pass it on further.

"The fact that the secondary infections have only very seldom passed on is comforting, but it's important to keep a close eye on things," Buchholz says.