When you study something that is, in the strictest sense, invisible, answers to seemingly straightforward questions quickly become elusive.

Like: What makes one virus replicate more efficiently than another? And why do some people get really, really sick—while others are able to fight off the same illness with few symptoms?

“Oh, there are lots of questions,” said Vincent Munster, the chief of the Virus Ecology Unit at the National Institute of Allergy and Infectious Disease, when I asked him to describe the sorts of things that still have virologists puzzled about the MERS virus. MERS refers to Middle East Respiratory Syndrome, a new and potentially deadly virus that has killed at least nine people and sickened dozens more in South Korea since last month, prompting travel warnings and a massive quarantine order. Symptoms can include fever, cough, and shortness of breath—just like many other less serious viruses, which makes MERS difficult to diagnose at first.

“Here's a very important question,” Munster said. “How does this virus spread so easily between healthcare settings?”

MERS has only been observed in humans since 2012, and the recent cases in South Korea represent the largest outbreak of the virus ever outside of Saudi Arabia, where it originated in camels before jumping to humans. “For dromedary camels, [MERS is] very much like what we get in a common cold,” Munster said. “But if it comes into humans, it moves into the lower respiratory tract where it can cause some harm. If you are relatively healthy, you probably don't get too sick from this virus. But if you have co-morbidities—let's say a heart condition, diabetes, or obesity, maybe all three—the outcome for you if you get this virus is increasingly worse.”

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?”

Maybe not. Early studies show there’s reason to believe that MERS poses less of a global threat than SARS did—which is good news for the general population—even though scientists don’t yet understand why. “MERS is not as transmissible as SARS," Pallansch told me. “That doesn't answer the question why, but it is an observation that can be used to public-health advantage in that infection control procedures would be expected to be effective in stopping MERS.”

Rigorous hygiene practices—hand-washing with soap and warm water, especially—are the best way for individuals to protect themselves. Health officials are also reminding people to avoid touching their eyes, nose, and mouth. “It does not reduce your risk to zero,” Pallansch said. But hand washing will also help guard against viruses that most people are far more likely to encounter, like the flu. (The CDC estimates thousands of people in the United States alone died from influenza this past winter.)

“I would say MERS is less concerning because it doesn't seem to affect the general population,” Munster said. “This virus seems to be able to jump a couple of times but then hopefully dies out again. But it is of concern in the healthcare setting—and that would be the same in the U.S. Hospitals have a lot of susceptible people inside. People should start thinking really carefully about hospital-hygiene practices and personal hygiene.”