MERS has been called "a threat to the entire world" by the director of the World Health Organization. The virus, which had previously just hung around Saudi Arabia, has recently made the leap to South Korea, triggering the largest-ever outbreak outside the Middle East. Since then, travelers to China and Thailand brought the virus to those countries for the first time.
In South Korea, 166 people have been infected, 24 have died, and thousands have been under quarantine or observation. As of mid-June the numbers of cases seemed to be leveling off, but the WHO warned that "outbreaks are unpredictable."
What made the South Korea situation so surprising is that MERS — which stands for Middle East Respiratory Syndrome — isn't supposed to spread easily among humans. Scientists had thought of it as a virus that primarily lives in animals and only seldom affects people. In the past, when MERS has made the leap to human populations, the disease hasn't gotten very far.
But as it turns out, there's a lot we still don't know about MERS. It was only discovered in 2012, and the countries that previously harbored the virus, like Saudi Arabia, haven't always been transparent about their findings. Experts still don't understand exactly how MERS is transmitted, which is worrisome. And the fact that the disease is a deadly respiratory infection, from the same family as SARS (which infected more than 8,000 people during a 2003 global pandemic), has health officials on guard.