Viruses are the smallest forms of life on earth. In fact, many scientists do not consider viruses to be alive at all, as they exist at the border between life and biochemistry. The smallest examples of viruses are around twenty nanometres in diameter (a nanometre is one millionth of a millimetre), which means that around eight billion viruses could be accommodated on a pinhead (presently there are six and a half billion people on earth).

Viruses are also quite simple, their genomes can contain as few as three genes, while the human genome is estimated to contain twenty-five thousand genes. This simplicity is reflected in their structure: viruses consist of a genome, which can be made from DNA or a similar molecule called RNA. This is enclosed within a protective shell made out of protein, called a capsid. They sometimes also have a membrane, which helps them to enter their host cells more easily. Virus capsids are highly symmetrical, and they are made from many identical copies of one or only a few types of protein and are described as being either 'spherical' or 'helical'

Viruses are too small to see using light microscopes, so scientists use electron microscopes, which allow much higher magnification. To learn more about how viruses assemble and interact with their hosts, computers can be used to solve the three-dimensional structures of viruses from electron microscope images.