An infectious agent which multiplies within a cell. Viruses are therefore no longer considered as real living beings, like bacteria for instance, because they are not autonomous: to reproduce, they have to use the cells of living organisms by turning them to their advantage. When a virus enters a cell, the cell’s resources are mobilised to ensure the multiplication of the virus. The new viruses enter other cells in turn and so forth. This process - a viral infection - is often accompanied by the death of the host cell. These cellular deaths are at the source of the appearance of pathologies (flu, for instance) in infected organisms. Viruses are eliminated by the immune system of the infected organisms.