Viruses
Coronaviruses are a group of viruses known for causing the common cold. They have a halo, or crown-like (corona) appearance when viewed under an electron microscope.
The common cold is a viral infection of the upper respiratory tract. The most commonly implicated virus is a rhinovirus (30–80%), a type of picornavirus with 99 known serotypes.[16][17] Other commonly implicated viruses include human coronavirus (≈15%),[18][19] influenza viruses (10–15%),[20] adenoviruses (5%),[20] human respiratory syncytial virus, enteroviruses other than rhinoviruses, and metapneumovirus.[21] Frequently more than one virus is present.[22] In total over 200 different viral types are associated with colds.[5]