Amyotrophic lateral sclerosis, the most common of five forms of motor neuron disease, is associated with echovirus (an enterovirus) infection of the central nervous system, and with retrovirus activity (it is not known whether this retrovirus activity arises from a human endogenous retrovirus, or from an exogenous retrovirus).
GuillainBarré syndrome is associated with the bacterium Campylobacter jejuni, and with the viruses cytomegalovirus and enterovirus.