ALS is a disease that attacks the nervous system in otherwise healthy adults.
Some people (such as Stephen Hawking) live with the disease for many years.
Most die within a few years of diagnosis.
The biological mechanisms behind ALS are not well understood. Some of this
has to do with the inherent complexity of the disease. Much more of it has
to do with the fact that ALS research is not very well funded.
An excellent source of information on ALS is the