SARS-CoV-2 (Severe Acute Respiratory Syndrome-related CoronaVirus-2) is a virus, belonging to the coronavirus family, that emerged in late 2019.

It is an enveloped single-stranded RNA virus. Its genome encodes numerous proteins, 4 of which correspond to structural proteins: a membrane protein (M), an envelope protein (E), a nucleocapsid protein (N) and a surface protein (Spike protein or S). The latter has sufficient affinity with the ACE2 and TMPRSS2 receptors to allow the virus to enter the cell. Because the nose and throat have cells with the necessary receptors for SARS-CoV-2, the virus enters the body and multiplies.

SARS-CoV-2, which causes Covid-19 (COronaVIrus Disease 2019), is the 7th virus capable of infecting humans.

The appearance of variants is linked to the mutation of viruses. After infecting our cells, viruses multiply by making copies of themselves. This process is not perfect and the copies may have "errors" that cause mutations. These mutations may have no consequences, or even a negative effect on the virus. Others may have an impact on the transmissibility of the virus or on the severity of the disease.