Pathogenic human-infecting viruses, illustration. From bottom left, clockwise, the virus particles are: influenza virus, norovirus, coxsackievirus, enterovirus D68, poliovirus, and adeno-associated virus. A virus is an infectious particle smaller than a bacterium and can only reproduce himself after infecting a host cell. They do not have their own metabolism and therefore rely on a host cell to replicate and propagate.