Coronaviruses are enveloped RNA viruses. The characteristic surface of a coronavirus virion has a crown-like appearance that can be seen under the electron microscope. ​

Most coronaviruses infect animals (i.e. bats, birds and mammals), which act as reservoirs and intermediate hosts, but can sometimes change host and infect humans.​

There are currently 7 coronaviruses known to infect humans. ​

• 4 of them cause mild-to-moderate disease: ​
  • HCoV-OC43, HCoV-HKU1 and HCoV-229E cause common colds​
  • HCoV-NL63 is an important cause of (pseudo) croup and bronchiolitis in children​
• 3 of them cause more severe and possibly even fatal disease: ​
  • SARS-CoV, responsible for severe acute respiratory syndrome (SARS) (2002)​
  • MERS-CoV, responsible for Middle East respiratory syndrome (MERS) (2012)​
  • SARS-CoV-2, responsible for coronavirus disease 2019 (COVID-19) (2019)​