ALS is extremely rare, affecting only about two out of every 100,000 people, according to the ALS Association [source: ALSA]. About 5,600 people are diagnosed with the disease in the U.S. each year, and between 20,000 and 30,000 people live with it at any one time [source: ALSA].
What exactly causes ALS isn’t known, but researchers are learning more about the disease all the time. In most cases (90 to 95 percent), there is no known cause, and the disease is called sporadic ALS. In the remaining cases (5 to 10 percent), the disease is passed down through families, and is called familial ALS. A child who is born to someone with familial ALS has a 50 percent chance of also getting the disease [source: ALSA].
Researchers have been looking for the gene or genes responsible for familial ALS, and in 1993, they made a huge breakthrough. They discovered a change or mutation in a gene called superoxide dismutase (SOD1). When the gene is normal, it directs the production of an enzyme that protects the motor neurons from damage caused by unstable molecules called free radicals. When the gene is mutated, the enzyme can’t protect the motor neurons, and they become damaged by the free radicals. About 20 percent of people with familial ALS have the SOD1 gene mutation. Researchers are still looking for more genes that might be involved.
Researchers also know that ALS is most common in Caucasian males, and it often starts when people are between the ages of 40 and 60 (although it can strike at an earlier age). People with ALS have too much of a substance called glutamate in their blood and spinal fluid. This chemical messenger helps nerve cells in the brain and spinal cord communicate with one another, but in large amounts it can actually damage motor neurons.
Researchers have speculated on several other possible causes of ALS, including:
However, there isn't enough evidence from scientific research to prove any of these theories.