Multiple sclerosis and viruses.

Discussing the problem of multiple sclerosis and viruses should not be limited to reviewing the epidemiological evidence in favor, or against, a particular candidate, such as Epstein-Barr virus or human herpes virus 6. In this text, I discuss the difficulty of going from association to causation in human epidemiology; the fact that viruses can trigger or prevent autoimmunity; the problem of our very limited knowledge of the viruses that we harbor as part of our metagenome; and the role of such viral flora in multifactorial diseases and also, possibly, in health.