A report released Tuesday by the World Health Organization laid out the circumstances surrounding the outbreak of Middle East Respiratory Syndrome (MERS), which killed more than 1,400 people in the Middle East between 2012 and 2015. The report noted a number of high-level investigations into the origins of the disease, leading most observers to conclude that the disease was spread by the simple-yet-effective stealth of infected animals. “More importantly, however, is the challenge posed by its emergence through a single source,” the report read. “This suggests that the predominant mode of infection may be animal-to-human spread. In such a case, the disease could be highly transmissible from animals to humans.”

The authors went on to call on the world to stockpile isolates of the disease to allow for a rapid response if it mutated and started to spread from humans to humans.

Just how contagious is MERS?

Jethro Pohorek, a professor at the University of Cambridge and leader of the team that carried out the first large-scale study of how the virus was spread, told Scientific American last month that there were “hundreds” of specimens found in animal reservoirs of the virus after an initial outbreak in 2012. (Unfortunately, some meat samples found after the initial outbreak have tested positive for MERS as well.)

While the virus has been able to survive outside the animals it originated in for longer than normal, further outbreaks are likely. The researchers who discovered MERS in 2012 wrote in the current report that at least 12 outbreaks of the disease in hospitals since then have happened, suggesting at least one way for the disease to spread from human to human.

How dangerous is it?

The current outbreak in the Middle East is the largest since MERS was first identified in 2012. It seems to be, however, predominantly animal-to-human spread rather than a crossover between human and animal hosts.

This is not necessarily the case in the outbreak of Ebola, another virus from the Congo. The illness has, indeed, been known to spread between human beings. According to the Inquisitr, a recent outbreak occurred in an area of the Congo where there are “areas that support the evolution of chimpanzees and other animal species, which are the reservoir hosts of the Zaire ebola virus.” The organization Doctors Without Borders, which runs a hospital near the affected area, notes on its website that the outbreak is “more infectious than Ebola and moves very quickly from person to person.”