Molecular model of surface sugars (blue) on a SARS (Severe acute respiratory syndrome) coronavirus spike (S) protein. The sugars, known as glycans, are derived from the host cell during viral replication and may play a role in helping the virus to evade the host's immune system. S proteins are found in the viral membrane, they bind to receptors on host cell membranes and facilitate the virus's entry to the cell. SARS is a viral respiratory illness caused by a coronavirus, called SARS-associated coronavirus (SARS-CoV).