Virus single CGI.  3d illustration of microscopic virus.  A virus is a submicroscopic agent that replicates only inside the living cells of an organism. Viruses interact with all life forms, from animals and plants to microorganisms, including bacteria and archaea.  Most viruses use endocytosis to enter the host cell.  A key step in COVID‐19 pathogenesis is the entry of the SARS‐CoV‐2 virus into the airway epithelial cells. Viruses have either DNA or RNA as their genetic material, protected by a protein coat known as a capsid.  RNA viruses are those that have ribonucleic acid (RNA) as their genetic material, while DNA viruses consist of deoxyribonucleic acid (DNA). DNA viruses are mostly double-stranded, while RNA viruses are single-stranded. The replication of DNA occurs in the nucleus and that of RNA takes place in the cytoplasm. Many viruses possess spike proteins that enable them to infect host cells and cause viral infections.