Viruses. Computer artwork of viruses (green) and a cell (red) that they are attacking. Viruses are minute non-cellular particles that invade living cells in order to reproduce. Viruses consist of a core of genetic material (a nucleic acid such as DNA) surrounded by a protein coat. Once inside the cell, they use the cell machinery to replicate themselves. The release of the replicates usually destroys the host cell. Viruses cause a wide range of diseases in plants and animals, including AIDS, influenza, measles and rabies.