Those co-morbidities—the fact that some pre-existing conditions make a person more likely to die from MERS—are a clue to Munster’s earlier question: Why is this virus moving so aggressively through hospitals?

Looking at the population most susceptible to severe illness from MERS may help explain why it is rampant in healthcare settings. The people who are at a greater risk of complications from MERS are more likely to be in the hospital in the first place. And, as The New York Times reported, the physically crowded hospital system in South Korea exacerbates the spread of viral illnesses. The way people go about getting admitted to the best hospitals in South Korea is part of the problem, the newspaper reported: Patients flock to big medical centers and wait in emergency rooms until they can be seen, potentially spreading germs in the process. Before the current outbreak, MERS cases worldwide had already started spiking up since last year. “The reason for this increase in cases is not yet completely known,” the CDC wrote in a statement on its website. “What CDC does know is that because we live in an interconnected world, diseases, like MERS, can make their way to the United States, even when they begin a half a world away.”

For scientists who are tracking the latest outbreak, examining the environment where MERS is spreading is only a piece of a larger puzzle. They’re still racing to understand the virus itself. Researchers know that, like other respiratory viruses, MERS is highly contagious because it is spread through droplets—from when a person coughs or sneezes, for instance. But other mechanics of how the virus behaves are a mystery.

“Our current knowledge is still rather imperfect,” said Mark Pallansch, the director of the Division of Viral Diseases at the Centers for Disease Control and Prevention’s National Center for Immunization and Respiratory Diseases. “Knowledge about viruses and other infectious agents is cumulative. So when you have exposure to diseases over many decades or even centuries, the doctors and scientists have learned more about those specific viruses than something that is brand new. When something is brand new, we often try to see what it is most similar to that we already know about. In the case of MERS, the immediate comparison was to SARS.”

SARS refers to Severe Acute Respiratory Syndrome, a virus that killed nearly 800 people and spread to dozens of countries in an huge outbreak in 2003. SARS and MERS are in the same larger family of viruses, called coronaviruses. Both viruses spread through the respiratory track and can cause severe illness. And both SARS and MERS can affect people and animals. “So by those close comparisons, you at least have a starting point for basic characteristics you can compare and evaluate the new agent to,” Pallansch said. “We have a start to better understanding MERS than if it had been something completely new and unknown. The next point: Is it going to be as bad as SARS?”