As of today (Jan. 20), a new respiratory virus in China has sickened more than 200 people across six cities. Cases have also been reported in South Korea, Japan, and Thailand. The head of the Chinese health authority confirmed that it appears the virus can be spread from person to person, putting health authorities on high alert ahead of the Lunar New Year, which is accompanied by some of the year’s biggest travel days in the region.

The first cases of the virus appeared in the mainland city Wuhan late last year. Health officials say the virus, which isn’t named yet, causes a fever and shortness of breath, which has led to pneumonia in some cases and three deaths so far.

Virologists studying this recent outbreak have identified the culprit as a coronavirus, which gets its name for the protein protrusions that decorate the outside of the virus like a crown. At first glance, coronaviruses are unremarkable: They’re common among animals, and in humans cause illnesses similar to the common cold, which are usually not severe. But coronaviruses have one trait that makes them particularly hard to contain: They’re zoonotic, meaning they have the ability to jump between animals and humans.

Viruses are essentially genetic code encapsulated in a protein shell, and they have three goals in life (if we can call them alive): Find another life form they’re compatible with, copy themselves like crazy, and jump into the next host. Typically, viruses are only capable of invading a certain type of cell, like for example those that line the respiratory tracts of pigs. That’s why viruses that make animals sick usually won’t make their way to humans.

That’s not the case with coronaviruses and other zoonotic microbes. These viruses have a single strand of genetic material, called RNA, which means it mutates easily as it copies itself. (More complex lifeforms, like us and animals, have double-stranded genetic material called DNA. The mirroring strand can act like a copy editor.) Most of the time, these mutations are useless—but every now and then, they can enable a pathogen to infect a new species, as Ben Longdon, a virologist at the University of Exeter, explained for TED Ed.

This particular virus seems to have originated from a fish and live-animal market in Wuhan, according to the World Health Organization. The coronavirus that caused the 2003 severe acute respiratory syndrome, or SARS, epidemic in China, originated in horseshoe bats before making its way to civets, small mammals that some people eat, which is how it became the first pandemic of the 21st century. The 2012 Middle East respiratory syndrome, or MERS, first jumped from camels to humans.

Zoonotic viruses are particularly dangerous because they’ve got multiple species to infect and spread between, and are therefore harder to contain. Our risk of picking up these types of viruses has only gone up in modern times, as we continue to interact with animals and encroach further into their territory through activities like deforestation. Climate change hasn’t helped, because warmer climates mean that certain pathogens can survive for longer in more parts of the world. And globalization means that we can introduce viruses to new parts of the world as we travel.

With quick action from public health officials, it’s still possible to contain viruses. Hong Kong has already initiated a disease response plan, and airports in Shenzhen have started screening passengers for fevers and are doubling down on measures to stop illegal wildlife trading. Hopefully, these coordinated efforts will be enough to stop this coronavirus from reaching around the globe.