Illustration of SARS-CoV-2 viruses, enveloped RNA (ribonucleic acid) viruses. SARS-CoV-2 causes the respiratory infection Covid-19, which can lead to fatal pneumonia. The surface of these viruses consists of a lipid membrane (yellow) and three protein types, embedded in the membrane. These protein types are: spike (S) protein trimers (red and purple) to which sugar molecules are attached (black), envelope (E) proteins (dark yellow), whose main body is hidden within the membrane and which protrudes a short distance outwards and dimers of the membrane (M) protein (dark yellow), which are also mainly hidden in the membrane and which end directly at the lipid surface. Spike proteins allow the virus to bind to the host cell. Membrane proteins and envelope proteins play a role in virus assembly.