THE new coronavirus is the latest example of a disease that jumped from animals into humans. When infections do this they can be deadly 鈥� and 2019-nCoV is no exception.
Nearly all viruses and bacteria that infect other organisms are completely harmless to people. But a tiny proportion can infect us and cause so-called zoonotic diseases, which come from animals rather than other people.
鈥淶oonotic diseases can be so deadly because we have no pre-existing immunity to them鈥�
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Such diseases are a massive problem. They make around 2.5 billion people ill every year and kill 2.7 million, . Not all zoonotic diseases cause serious illnesses, but the Ebola virus, for example, currently kills most of those it infects.
One reason zoonotic viruses can be this deadly is that we lack pre-existing immunity to them. Another is that these viruses aren鈥檛 adapted to humans. Viruses that normally circulate among people can evolve to become less lethal, as this helps them spread. 鈥淭hey don鈥檛 want you to drop dead within a day because you won鈥檛 pass it to anyone else,鈥� says Chris Coleman at the University of Nottingham, UK.
To get infected, people need to come into contact with the animal the virus usually infects. This is most likely with domesticated animals. Camels carry the MERS coronavirus that causes sporadic human cases, for instance.
Many viruses that jump into people, like MERS, seldom spread from person to person. They can still infect thousands, though: rabies is mostly passed on by dog bites, but kills 60,000 people a year. Others, such as Ebola, can spread from person to person, but aren鈥檛 very good at it and so cause relatively small outbreaks.
The 2019 coronavirus, by contrast, appears quite good at spreading from person to person. While we don鈥檛 know how deadly it is yet, Coleman says 鈥渋t鈥檚 not the most deadly coronavirus we鈥檝e ever had鈥�.
To trace the new virus鈥檚 origins, researchers have been comparing its genome to that of other coronaviruses. This showed that it derives from a strain that infects bats, possibly the intermediate horseshoe bat (Nature, ). 鈥淚t鈥檚 highly related to bat coronaviruses,鈥� says Vaithilingaraja Arumugaswami at the University of California, Los Angeles.
This makes sense as bats are known to harbour many viruses, including coronaviruses. Viral infections are especially likely to spread among bats as they can fly long distances and roost close to each other. Bats also seem able to carry infections without getting ill, which helps spread the viruses.
The new coronavirus might have jumped from bats into another animal a few months or even a few decades ago, and then from that intermediate host into humans. We know that the coronavirus behind the SARS outbreak of 2002 to 2003 spread before infecting people.
One initial study suggested the new virus could have come from snakes, but some biologists are sceptical. Another early study implicated mink. Samples from animals at the Wuhan market where the virus appears to have emerged are being tested, but no results have been announced.
It took years to trace the origins of SARS, but technology has greatly improved since then. In the meantime, some charities have called for the permanent closure of markets in China selling wild animals. But this could lead to a more dangerous black market trade. When China shut bird markets in 2013 and 2014 to try to stop H7N9 bird flu spreading, it .
Coleman thinks there is little we can do to stop people coming into contact with animals that may carry dangerous viruses. 鈥淚t鈥檚 very difficult to control that,鈥� he says.
Instead, he says we need to have vaccines ready in advance. This could mean creating vaccines that are effective against a wide range of viruses or developing vaccines that require only minor tweaks to work against a new viral strain, much like annual flu vaccines.
Other infections that have jumped to humans
Biologists have been warning for decades about the risks of animal viruses spreading to people, and calling for more surveillance and preparation.
There is good reason to worry. The last global pandemic, the 2009 flu that killed up to 400,000 people, was caused by a strain of flu that came from pigs. And that flu is thought to be a descendant of the 1918 flu, which came from birds and which killed up to 50 million people.
HIV, which has infected about 75 million people, is now thought of as a human virus. But it jumped from chimpanzees into humans relatively recently, in the 1930s.
There are already around four human coronaviruses, which usually cause mild, cold-like symptoms. They are thought to have come from animals thousands of years ago.
Flu remains one of the biggest dangers, with fears that a very deadly strain could emerge. Other zoonotic threats include Lassa fever, Rift Valley fever, Marburg virus disease and Nipah virus infection.
