Only in a few cases and after many years does it cause demyelination.Evidence for a virus as a cause include the presence of oligoclonal bands in the brain and cerebrospinal fluid of most people with MS, the association of several viruses with human demyelination encephalomyelitis, and the occurrence of demyelination in animals caused by some viral infections.The T cells recognize myelin as foreign and attack it, explaining why these cells are also called "autoreactive lymphocytes".
Gout occurs less than would be expected and lower levels of uric acid have been found in people with MS.
This has led to the theory that uric acid is protective, although its exact importance remains unknown.
Uhthoff's phenomenon, a worsening of symptoms due to exposure to higher than usual temperatures, and Lhermitte's sign, an electrical sensation that runs down the back when bending the neck, are particularly characteristic of MS.
The main measure of disability and severity is the expanded disability status scale (EDSS), with other measures such as the multiple sclerosis functional composite being increasingly used in research.
Fitting with an immunological explanation, the inflammatory process is caused by T cells, a kind of lymphocyte that plays an important role in the body's defenses.
T cells gain entry into the brain via disruptions in the blood–brain barrier.
The hygiene hypothesis proposes that exposure to certain infectious agents early in life is protective, the disease being a response to a late encounter with such agents.
The prevalence hypothesis proposes that the disease is due to an infectious agent more common in regions where MS is common and where in most individuals it causes an ongoing infection without symptoms.
Environmental factors may play a role during childhood, with several studies finding that people who move to a different region of the world before the age of 15 acquire the new region's risk to MS.