Multiple sclerosis and viruses

Multiple sclerosis is an autoimmune disorder that causes the loss of myelin from axons in the brain and spinal cord. Patients with multiple sclerosis develop attacks of focal neurological dysfunction, and the deficits they experience depend on the location of the inflammatory lesions that occur. One hypothesis to explain why the immune system attacks cells of the central nervous system is that an unknown virus may be infecting the cells, causing immune cells to kill those cells.

A researcher has identified three viruses that she hypothesizes may be involved. To test this hypothesis, she administers the viruses to different groups of mice. She then repeatedly examines the mice for neurological deficits, and analyses sections of their brain tissue for changes to various cell populations. The mice that receive virus one develop neurological deficits but no noticeable changes in the brain tissue. The mice that receive virus two develop neurological deficits and inflammatory lesions in the brain tissue. The mice that receive virus three develop neither neurological deficits nor noticeable changes in the brain tissue. However, the researcher is surprised to find that the control mice also develop neurological deficits and inflammatory brain lesions, when they were expected to remain healthy.