Coronavirus is a virus in the family Coronaviridae. They are enveloped viruses with a positive-sense RNA genome. They have a nucleocapsid of helical symmetry. The genome size of coronaviruses is about 26 to 32 kilobases, which is extraordinarily large for an RNA virus. They infect various kinds of mammals. There are four major groups of coronaviruses, called alpha, beta, gamma and delta. The most famous coronavirus is one of the betas, the kind that causes Coronavirus disease 2019 in humans.