Coronaviruses are a large family of viruses. Many of them infect animals, but some coronaviruses from animals can evolve or change into a new (novel) human coronavirus that can spread from person-to-person. This is what happened with the new coronavirus known as SARS-CoV-2, which causes the disease known as COVID-19. Diseases from coronaviruses in people typically cause mild to moderate illness, like the common cold. Some, like the SARS or MERS viruses cause serious infections like pneumonia.