You may have heard about coronaviruses recently. Unfortunately, you'll probably be hearing a lot more. They're a type of virus that normally infects only animals, but they are now starting to infect people. And they can kill.

Right now, one coronovirus, dubbed 2019 novel coronovirus or 2019-nCoV, is causing an outbreak of pneumonia in China. As of late January, over 2,000 cases have been confirmed, with 56 deaths. Cases have been reported in 13 locations outside of China, including five cases in the United States, all in people who had travelled to Wuhan, China.

It's concerning when a virus makes the jump from animals to humans. With little previous exposure to it, people haven't had a chance to build up immunity.

The CDC describes the immediate health risk to the general American public as “low at this time.” However, there is a great deal of uncertainty surrounding the outbreak and the situation remains fluid. It's not clear yet how easily or quickly this virus is spreading between people, but when a virus makes the jump from infecting animals to infecting people, it is always concerning. People have had little previous exposure to the virus and therefore little chance to build up immunity to it.

Because most cases of the current coronavirus are occurring far away, in China, the outbreak may not yet be on the radar among people in Europe and the United States. Yet the strains of virus responsible for the yearly flu outbreak also come to the United States from far away. With international air travel, viruses can spread far and fast.

Coronaviruses have been around for a long time but have largely been ignored because they mostly infect animals. This changed in 2002, when a severe, unusual outbreak of pneumonia began in Guangdong province in China. It was eventually brought under control, but not before more than 8,000 people were infected and over 750 died. Most cases were in Hong Kong and China. Twenty to 30 percent of infected people required mechanical ventilation and 10 percent died, with higher fatality rates in older patients and those with other medical issues. The disease was named SARS, short for severe acute respiratory syndrome.

While the virus did spread from person to person, its natural reservoir appears to be in bats. Somehow, it had managed to jump from bats to people.

In 2012, a different coronavirus made the jump from animals to humans, this time in Saudi Arabia, causing another epidemic of respiratory disease. The virus was distinct from the one that caused SARS and the disease that it caused was named Middle East respiratory syndrome (MERS). Nearly 2500 cases and over 800 deaths were reported, with most cases coming in Saudi Arabia. The virus is presumed to normally live in bats, though there have been apparent cases of transmission from dromedary camels.

While the SARS and MERS epidemics did spread beyond their original location, most cases occurred close to home. Of course, every outbreak is unique and the current one is fairly new, but if it runs like the other two, the U.S. may be spared the worst of its fury, though with three different coronavirus outbreaks in the last 20 years or so, it's likely that there will be more.

At the moment there's little that people can do to protect themselves from a potential spread of the new coronavirus. The CDC is issuing periodic updates on the situation and while they are concerned, they see no need for panic.

A Viewpoint by Anthony S. Fauci, MD, Director of the National Institute of Allergy and Infections Diseases (NIAID)i, detailing all three coronavirus outbreaks has recently been published in JAMA.