‘What are viruses?’ introduces viruses and their structure. Martinus Beijerinck, in 1898, was the first to coin the term ‘virus’, and invention of the electron microscope in the late 1930s greatly enhanced virus identification. Viruses are not cells, but obligate parasites that must infect a cell and use its organelles in order to reproduce. They carry either DNA or RNA, and have a protein coat called a capsid. The whole structure is called a virion. Viruses have a high mutation rate, which helps them to survive and boost their resistance to antiviral drugs. The molecular clock technique to track a virus’s history is also explained.