Coronaviruses are a family of RNA viruses that infect and cause disease in mammals and birds. In humans they typically cause upper respiratory infections that can range from mild to fatal.
Coronaviruses constitute the subfamily Orthocoronavirinae, in the family Coronaviridae. They have characteristic club-shaped spikes that project from their surface, which in electron micrographs create an image reminiscent of the stellar corona, from which their name derives.
Alphacoronaviruses and betacoronaviruses infect mammals, while gammacoronaviruses and deltacoronaviruses primarily infect birds.
Betacoronaviruses (β-CoVs or Beta-CoVs) are one of four distinct genera of coronavirus. Member viruses are enveloped, positive-strand RNA viruses that infect mammals (including humans). The natural reservoir for betacoronaviruses are bats and rodents. Rodents are the reservoir for the subgenus Embecovirus, while bats are the reservoir for the other subgenera.
Each coronavirus genera are composed of various viral lineages with the betacoronavirus genus containing four such lineages: A, B, C, and D.
Severe acute respiratory syndrome-related coronaviruses (SARS-CoV) are a species of coronavirus (family Coronaviridae) within the genus Betacoronavirus and the subgenus Sarbecovirus (ie. SARS Betacoronavirus).
Bats serve as the main host reservoir species for the SARS-related coronaviruses. While SARS-coronaviruses have evolved in bats for thousands of years, they have only recently evolved to be able to jump into other species, including humans.
There have been two strains of SARS-CoV that have made the jump into humans. Both have caused significant outbreaks. The first was SARS-CoV-1 (causes the disease referred to as SARS) in 2002-2004. The second was SARS-CoV-2 (causes the disease known as COVID-19), which since its arrival in late 2019 has become the largest pandemic in modern history.
Both SARS-CoV-1 and SARS-CoV-2 strains descended from a single ancestor but made the cross-species jump into humans separately. SARS-CoV-2 is not a direct descendant of SARS-CoV-1.
COVID-19 infections are caused by a virus called SARS-CoV-2. It is part of the coronavirus family, which include common viruses that cause a variety of diseases from head or chest colds to more severe (but more rare) diseases like severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).
Like many other respiratory viruses, coronaviruses spread quickly through droplets that you project out of your mouth or nose when you breathe, cough, sneeze, or speak.
The word corona means crown and refers to the appearance that coronaviruses get from the spike proteins sticking out of them. These spike proteins are important to the biology of this virus. The spike protein is the part of the virus that attaches to a human cell to infect it, allowing it to replicate inside of the cell and spread to other cells. Some antibodies can protect you from SARS-CoV-2 by targeting these spike proteins. Because of the importance of this specific part of the virus, scientists who sequence the virus for research constantly monitor mutations causing changes to the spike protein through a process called genomic surveillance.
As genetic changes to the virus happen over time, the SARS-CoV-2 virus begins to form genetic lineages. Just as a family has a family tree, the SARS-CoV-2 virus can be similarly mapped out. Sometimes branches of that tree have different attributes that change how fast the virus spreads, or the severity of illness it causes, or the effectiveness of treatments against it. Scientists call the viruses with these changes “variants”. They are still SARS-CoV-2, but may act differently.