Yes it is rare i believe 1 or 2 people in 100,000 people can suffer with it, there are familial types too that can be passed down that increase your risk, men are more at risk than women i think but it is even more rare to get it if you are under 40 but some neurologists believe that extreme sports people have a higher chance of developing this horrible condition.
@ Chris - I also wanted to add that twitching in ALS happens AFTER weakness and you would notice a limb doesn't work properly not before and it is usually in 1 area and constant, not like the twitches we get.
By all means see a neuro if it will put your mind at rest but please try not to jump to other diseases otherwise this will be never ending for you.
Has your doctor put you on any meds and/or therapy?