Virus?

Someone on the ALS Yahoo group sent an article about a hypothesis that viruses cause ALS.

In my opinion, the hypothesis is not supported by epidemiological evidence. There is no indication that people catch ALS from each other. The incidence is sporadic, with no worthwhile clustering. For a virus to cause ALS, this would have to be a virus that is so common, so highly transmissible, from people or the environment, that virtually everyone has it present in their bodies. Then the question is still the same: Why do some people get sporadic (non-inherited) ALS, and not others? Why do so many people NOT get ALS? In hypothesizing that a virus causes ALS, we are hypothesizing that something causes ALS, but that we don't know what that something is.