The onset of an acute demyelinating disease (postinfectious encephalomyelitis) occurs after a number of different infections such as measles and mumps as well as smallpox vaccination. About one quarter of these cases diagnosed as postinfectious encephalomyelitis end up with a diagnosis of MS. This naturally raised the theory that viruses might be the cause of MS.
Over the years, research has implicated many infectious agents, such as the measles virus (and other paramyxoviruses), distemper, the T-cell leukemia virus, as well as certain bacteria, as possible environ-mental factors. For the most part, they have been dis-carded. No single infection is known to cause MS.