For almost a decade, scientists chased  the source of a deadly new virus through China’s tallest mountains  and most isolated caverns. They finally found it here:  in the bats of Shitou Cave. The virus in question was a coronavirus that caused an epidemic  of severe acute respiratory syndrome, or SARS, in 2003. Coronaviruses are a group of viruses covered in little protein spikes  that look like a crown— or "corona" in Latin. There are hundreds  of known coronaviruses. Seven of them infect humans,  and can cause disease. The coronavirus SARS-CoV causes SARS,  MERS-CoV causes MERS, and SARS-CoV-2 causes  the disease COVID-19. Of the seven human coronaviruses,  four cause colds, mild, highly contagious infections  of the nose and throat. Two infect the lungs,  and cause much more severe illnesses. The seventh, which causes COVID-19,  has features of each: it spreads easily,  but can severely impact the lungs. When an infected person coughs,  droplets containing the virus spray out. The virus can infect a new person when the droplets enter their nose or mouth. Coronaviruses transmit best  in enclosed spaces, where people are close together. Cold weather keeps their delicate casing from drying out, enabling the virus to survive  for longer between hosts, while UV exposure from sunlight  may damage it. These seasonal variations matter more  for established viruses. But because no one is yet immune  to a new virus, it has so many potential hosts that it doesn’t need ideal conditions to spread. In the body, the protein spikes embed in the host’s cells and fuse with them— enabling the virus to hijack  the host cell’s machinery to replicate its own genes. Coronaviruses store their genes on RNA. All viruses are either RNA viruses  or DNA viruses. RNA viruses tend to be smaller,  with fewer genes, meaning they infect many hosts  and replicate quickly in those hosts. In general, RNA viruses don’t have  a proofreading mechanism, whereas DNA viruses do. So when an RNA virus replicates, it’s much more likely to have  mistakes called mutations. Many of these mutations are useless  or even harmful. But some make the virus better suited for certain environments— like a new host species. Epidemics often occur when a virus  jumps from animals to humans. This is true of the RNA viruses  that caused the Ebola, Zika, and SARS epidemics,  and the COVID-19 pandemic. Once in humans, the virus still mutates— usually not enough to create a new virus, but enough to create variations,  or strains, of the original one. Coronaviruses have a few key differences  from most RNA viruses. They’re some of the largest,  meaning they have the most genes. That creates more opportunity  for harmful mutations. To counteract this risk,  coronaviruses have a unique feature: an enzyme that checks for replication  errors and corrects mistakes. This makes coronaviruses  much more stable, with a slower mutation rate,  than other RNA viruses. While this may sound formidable, the slow mutation rate  is actually a promising sign when it comes to disarming them. After an infection, our immune systems  can recognize germs and destroy them more quickly  if they infect us again so they don’t make us sick. But mutations can make a virus  less recognizable to our immune systems— and therefore more difficult to fight off. They can also make antiviral drugs  and vaccines less effective, because they’re tailored  very specifically to a virus. That’s why we need a new flu vaccine every year— the influenza virus mutates so quickly  that new strains pop up constantly. The slower mutation rate  of coronaviruses means our immune systems, drugs,  and vaccines might be able to recognize them  for longer after infection, and therefore protect us better. Still, we don’t know how long our bodies remain immune to different coronaviruses. There’s never been an approved treatment or vaccine for a coronavirus. We haven’t focused on treating  the ones that cause colds, and though scientists began developing treatments for SARS and MERS, the epidemics ended before those  treatments completed clinical trials. As we continue to encroach  on other animals’ habitats, some scientists say a new coronavirus  jumping to humans is inevitable— but if we investigate these unknowns,  it doesn’t have to be devastating.