The World Health Organization has announced three new deaths from a new virus that has alarmed health officials in the Middle East and Europe since it was first discovered last year.

The organization has now confirmed 50 cases of what it calls Middle East respiratory syndrome, or MERS. Thirty of those patients have died.

Those numbers are cause for concern. The MERS virus is a coronavirus, which is the same type of virus responsible for the common cold as well as SARS, but less than a tenth of the people who fell ill in the SARS epidemic of 2002 and 2003 died. The new avian influenza virus spreading in China, known as H7N9, has resulted in death in a quarter of known cases, while the more well-known bird flu strain, H5N1, kills about half of those who are infected, according to The New York Times. It's still too early to say whether the MERS virus is more dangerous than these other viruses, since many more people could have been infected without developing severe symptoms.

Yet MERS is worrisome for other reasons. A study published online Thursday in The Lancet described how one patient in a French hospital caught the disease from another who had recently traveled to Dubai and did not survive. The two shared a room together for three days, but it wasn't until more than a week afterward that symptoms appeared in the other patient. The authors of the study estimated the incubation period at 9-12 days, a long time in which infected people could spread the disease without realizing that they are ill. It still isn't certain at what stage people carrying the virus are most contagious, the Los Angeles Times reports.

Another concern is that the virus is difficult to identify. An article published in The New England Journal of Medicine this week described how the disease apparently spread through three generations of a family in Riyadh. A boy and his uncle survived the illness, while the boy's father and grandfather did not. Yet doctors could not confirm the presence of the MERS virus in the youngest of the patients, and were not able to confirm it in the two who died until after they passed away.

The World Health Organization is not advising restrictions or special screening for travelers, but it has issued recommendations for surveillance that describe what respiratory illnesses hospital personnel should regard as suspicious. Still, the agency has to rely on doctors to find cases, who in many countries have other problems to worry about, as Foreign Policy reports. For example, the virologist who first identified the MERS pathogen at a Saudi hospital told the Guardian that he was fired for making his discovery public.

We do not know yet how dangerous the virus is, but we can hope health care systems around the world are up to the task of monitoring and preventing infections, in which case we won't have to find out.