ALS is short for amyotrophic lateral sclerosis. You might also have heard it called Lou Gehrig’s disease, after the baseball player who was diagnosed with it in the 1930s. A French doctor named Jean-Martin Charcot discovered the condition in 1869.

ALS is a progressive disease, which means it gets worse over time. It affects nerves in your brain and spinal cord that control your muscles. As your muscles get weaker, it gets harder for you to walk, talk, eat, and breathe.

It’s a disease that affects your motor neurons. These nerve cells send messages from your brain to your spinal cord and then to your muscles. You have two main types:

• Upper motor neurons: Nerve cells in the brain.
• Lower motor neurons: Nerve cells in the spinal cord.

These motor neurons control all your voluntary movements — the muscles in your arms, legs, and face. They tell your muscles to contract so you can walk, run, pick up your smartphone, chew and swallow food, and even breathe.

ALS is one of a few motor neuron diseases. Some others include:

• primary lateral sclerosis (PLS)
• progressive bulbar palsy (PBP)
• pseudobulbar palsy

With ALS, motor neurons in your brain and spinal cord break down and die.

When this happens, your brain can’t send messages to your muscles anymore. Because the muscles don’t get any signals, they become very weak. This is called atrophy. In time, the muscles no longer work and you lose control over their movement.

At first, your muscles get weak or stiff. You may have more trouble with fine movements — such as trying to button a shirt or turn a key. You may stumble or fall more than usual. After a while, you can’t move your arms, legs, head, or body.

Eventually, people with ALS lose control of their diaphragm, the muscles in the chest that help you breathe. Then they can’t breathe on their own and will need to be on a breathing machine.

There are two types of ALS:

• Sporadic ALS is the most common form. It affects up to 95% of people with the disease. Sporadic means it happens sometimes without a clear cause.
• Familial ALS (FALS) runs in families. About 5% to 10% of people with ALS have this type. FALS is caused by changes to a gene. Parents pass the faulty gene to their children. If one parent has the gene for ALS, each of their children will have a 50% chance of getting the gene and having the disease.

Researchers still don’t know exactly what causes motor neurons to die with ALS. Gene changes, or mutations, are behind 5% to 10% of ALS cases. More than 12 different gene changes have been linked to ALS.

One change is to a gene that makes a protein called SOD1. This protein may be toxic to motor neurons. Other gene changes in ALS might also damage motor neurons.

Environment could also play a role in ALS. Scientists are studying whether people who come into contact with certain chemicals or germs are more likely to get the disease. For example, people who served in the military during the 1991 Gulf War have gotten ALS at higher rates than usual.

Scientists are also looking at these other possible causes:

• Glutamate. This chemical sends signals to and from the brain and nerves. It’s a type of neurotransmitter. With ALS, glutamate builds up in the spaces around nerve cells and may damage them.
  The only medications approved to treat ALS are riluzole (Rilutek) and edaravone (Radicava). They work by lowering glutamate levels.
• Immune system problems. Your immune system protects your body from foreign invaders such as bacteria and viruses. In your brain, microglia are the main type of immune cell. They destroy germs and damaged cells.
  With ALS, microglia might also destroy healthy motor neurons.
• Mitochondria problems. Mitochondria are the parts of your cells where energy is made. A problem with them might lead to ALS or make an existing case worse.
• Oxidative stress. Your cells use oxygen to make energy. Some of the oxygen your body uses to make energy may form into toxic substances called free radicals, which can damage cells.

Researchers learn more about ALS every day. What they discover will help them develop medications to treat symptoms and improve the lives of people who have this disease.