How ALS develops in your body

ALS happens when the nerve cells that control your muscles gradually die. These cells, called motor neurons, sit in your brain and spinal cord and send signals that tell your muscles to move. When motor neurons break down, your muscles stop receiving those signals. They weaken, shrink, and eventually stop working altogether. You lose the ability to move, speak, swallow, and breathe on your own.

The damage happens over months or years. Early on, you might notice weakness in one hand, foot, or your speech. Over time, the weakness spreads to other parts of your body. Most people with ALS live two to five years after symptoms start, though some live longer. The disease does not affect your mind or your senses—you remain aware and able to think clearly.

Key Takeaways

  • ALS kills motor neurons in the brain and spinal cord, which stops muscles from receiving movement signals.
  • About 90 percent of ALS cases have no known genetic cause; the remaining 10 percent run in families and are inherited.
  • Scientists have found several gene mutations linked to ALS, but inheriting a mutation does not may provide you will develop the disease.
  • Risk factors include being male, being between 40 and 70 years old, and having a family history of ALS or frontotemporal dementia.
  • No single cause has been proven; researchers believe multiple factors—genes, environment, and aging—work together to trigger the disease.

The two types of ALS: inherited and sporadic

About 90 percent of ALS cases are sporadic, meaning they appear randomly with no family history. These cases are not passed down from a parent. The other 10 percent are familial ALS, or fALS, which runs in families. If one of your parents has familial ALS, you have a 50 percent chance of inheriting the gene mutation that causes it.

Inheriting the mutation does not mean you will definitely develop ALS. Some people carry the mutation their whole lives and never get sick. Others develop symptoms in their 30s; others not until their 70s. Scientists do not yet understand why the disease appears in some carriers and not others, or why it strikes at different ages.

Sporadic ALS is harder to explain because there is no family pattern to follow. Researchers believe that in these cases, a combination of genetic risk, environmental exposures, and aging all contribute. A single gene mutation alone does not seem to cause sporadic ALS the way it does familial ALS.

Gene mutations linked to ALS

Scientists have identified more than 30 genes where mutations are associated with ALS. The most common is a mutation in the C9orf72 gene, which accounts for about 5 to 10 percent of familial cases in North America and Europe. Other well-studied genes include SOD1, FUS, and TARDBP. Each mutation affects motor neurons in slightly different ways, but the end result is the same: the cells die and muscles weaken.

Finding a mutation in your genes does not tell you when or whether you will develop ALS. It is a risk factor, not a diagnosis. Some people with known ALS mutations never show symptoms. Researchers are still working to understand what determines whether a mutation will cause disease in a particular person.

Genetic testing can identify some mutations, but not all. If you have a family history of ALS, a neurologist can discuss whether testing makes sense for you. Testing is most useful for family members of someone already diagnosed, because it can show who carries the mutation and who does not.

Environmental and lifestyle factors under investigation

Researchers have looked for environmental triggers—things in your surroundings or habits that might cause ALS. Studies have examined pesticide exposure, heavy metals, smoking, military service, and intense physical activity. Some studies suggest links between certain exposures and ALS risk, but no single environmental cause has been proven to start the disease.

The strongest environmental finding involves military service. Veterans, particularly those who served in the Gulf War, have higher rates of ALS than the general population. The reason is not clear—it could be related to exposures during service, stress, or something else entirely. Researchers continue to investigate what about military service increases risk.

Smoking has been associated with higher ALS risk in some studies. Physical trauma and intense athletic activity have also been examined, with mixed results. The challenge is that ALS is rare, so it is hard to find enough cases to prove that any single factor causes it. Most likely, ALS requires multiple factors working together over time.

Age and sex as risk factors

ALS is more common in people between 40 and 70 years old, with the average age of diagnosis around 60. It can strike younger people—some are in their 20s or 30s—but this is less common. The disease becomes more likely as you age, which suggests that aging itself plays a role in motor neuron death.

Men are diagnosed with ALS about 1.5 times more often than women. Scientists do not know why. It could be biological—something about male physiology makes motor neurons more vulnerable. It could be that men are more likely to be exposed to environmental risk factors, or that women are underdiagnosed. Research into sex differences in ALS is ongoing.

How motor neurons fail: what happens at the cellular level

Inside motor neurons, proteins build up in clumps. The most common protein involved is called TDP-43. In healthy neurons, TDP-43 stays in the nucleus (the cell's control center) and helps manage other proteins. In ALS, TDP-43 leaks out of the nucleus and accumulates in the cell body, where it tangles and damages the neuron. The cell eventually dies.

Another protein, called SOD1, can also misfold and clump in some ALS cases. When proteins misfold, they trigger a chain reaction: the cell's machinery cannot clear the clumps, stress builds up, and the neuron dies. This process happens slowly over years, which is why ALS symptoms develop gradually rather than all at once.

Inflammation also plays a role. As motor neurons die, immune cells in the brain and spinal cord become activated and release inflammatory chemicals. This inflammation can speed up the death of remaining motor neurons. Some ALS treatments aim to reduce this inflammation, though so far no treatment has stopped the disease entirely.

Why ALS is hard to predict and prevent

Even when scientists find a gene mutation or an environmental exposure linked to ALS, they cannot predict who will get sick. This is because ALS almost certainly requires multiple factors—not just one gene or one exposure. A person might carry an ALS gene mutation, live in an area with pesticide use, smoke, and be male, yet never develop the disease. Someone else with fewer risk factors might develop ALS in their 50s.

This complexity is why there is no proven way to prevent ALS. You cannot change your age, sex, or genes. You can avoid smoking and reduce exposure to known toxins, but these steps do not may provide protection. If you have a family history of ALS, talking with a genetic counselor can help you understand your personal risk and what testing might reveal.

Frequently Asked Questions

Can you catch ALS from someone else?

No. ALS is not contagious. You cannot catch it from contact with someone who has the disease. The only way to inherit ALS is if you are born with a gene mutation that causes familial ALS, which happens only if one of your parents carries the mutation.

If my parent has ALS, will I definitely get it?

Not necessarily. If your parent has familial ALS, you have a 50 percent chance of inheriting the mutation. But inheriting the mutation does not may provide you will develop symptoms. Some people with the mutation never get sick, and others develop ALS at very different ages. Genetic counseling can help you understand your specific situation.

Does exercise cause ALS?

No. Exercise does not cause ALS. Some studies have looked at whether intense athletic activity increases risk, with unclear results. Regular physical activity is beneficial for your health and is safe for people without ALS symptoms. If you have been diagnosed with ALS, your doctor can advise on safe exercise.

Can a blood test show if I will get ALS?

Not yet. Genetic testing can identify some ALS mutations, but carrying a mutation does not mean you will develop the disease. Blood tests cannot predict whether you will get ALS or when symptoms might start. A neurologist or genetic counselor can discuss whether genetic testing is useful for you based on your family history.

What is the difference between ALS and other motor neuron diseases?

ALS affects both upper motor neurons (in the brain) and lower motor neurons (in the spinal cord). Other motor neuron diseases affect only one group. Primary lateral sclerosis affects only upper motor neurons; progressive muscular atrophy affects only lower motor neurons. These distinctions matter because they affect how the disease progresses and how it is treated.