ALS is not something you catch or inherit in a simple way

ALS (amyotrophic lateral sclerosis) develops when nerve cells in your brain and spinal cord gradually stop working. You do not get ALS from another person, and most people who develop it have no family history of the disease. About 90 percent of cases are sporadic—meaning they appear randomly, with no known cause passed down through your family. The remaining 10 percent are familial ALS, which means the disease runs in your family because of a genetic mutation one of your parents carried.

Scientists do not yet know exactly why nerve cells die in ALS. Research points to several things that may play a role: problems with how cells handle a protein called SOD1, inflammation in the nervous system, problems with how cells use energy, and exposure to certain environmental factors. The disease usually starts between ages 40 and 70, though it can occur at any age. Men are slightly more likely to develop it than women, but ALS affects both.

Key Takeaways

  • Most ALS cases (90 percent) are sporadic, meaning they occur randomly without a family history of the disease.
  • Familial ALS accounts for about 10 percent of cases and is caused by a genetic mutation inherited from a parent.
  • Scientists have identified several possible factors in nerve cell death, but no single cause has been proven for all cases.
  • ALS is not contagious and cannot be caught from another person or transmitted through contact.
  • The disease typically begins in middle age or later, though it can develop at any point in life.

What happens in your body when ALS develops

ALS damages motor neurons—the nerve cells that send signals from your brain and spinal cord to your muscles. As these neurons weaken and die, your muscles stop receiving the signals they need to contract. This causes weakness that usually starts in one area (your hands, feet, or speech) and spreads over time. You may notice you drop things, trip more often, or slur your words before you realize something is wrong.

The disease progresses at different speeds in different people. Some people lose muscle strength over months; others over years. ALS does not affect your senses, memory, or thinking ability in most cases, which means you remain aware of what is happening as your body changes. This is one reason why an ALS diagnosis is so difficult—your mind stays sharp while your physical abilities decline.

Sporadic ALS: the most common form

Sporadic ALS accounts for roughly 9 out of every 10 cases. It appears without warning and without a family history. Researchers have found that people with sporadic ALS sometimes carry mutations in genes like C9orf72, SOD1, or TARDBP, but these mutations are not inherited—they occur randomly in that person's cells. Environmental exposures may also increase risk, though no single exposure has been proven to cause the disease.

Because sporadic ALS has no clear cause, there is no known way to prevent it. If you develop sporadic ALS, it is not because of anything you did or did not do. Your family members are not at higher risk simply because you have the disease, unless genetic testing reveals a mutation that could be passed on.

Familial ALS: when the disease runs in families

Familial ALS is caused by a mutation in one of several genes. The most common is the C9orf72 gene, followed by SOD1 and FUS. If one of your parents carries a mutation, you have a 50 percent chance of inheriting it. However, inheriting the mutation does not may provide you will develop ALS—some people carry the mutation and never show symptoms, a phenomenon called incomplete penetrance.

If you have a family history of ALS, genetic testing can tell you whether you carry a mutation. This information may help you and your doctor watch for early signs and plan ahead. Some people choose testing; others do not. There is no right answer, and the decision is personal. Genetic counselors can help you think through what testing means for you and your family.

Risk factors and what researchers are still studying

Certain factors appear more often in people with ALS, though none of them cause the disease outright. These include being male, being over 40, military service (possibly related to environmental exposures), and smoking. Some studies suggest that head injury, exposure to pesticides or heavy metals, or intense physical activity may increase risk, but the evidence is not conclusive.

Researchers continue to investigate why some people develop ALS and others do not. Studies are underway looking at viral infections, immune system problems, and how the body handles toxic proteins. Understanding these mechanisms may eventually lead to ways to slow or stop the disease, but that research is still ongoing.

Why early diagnosis matters

If you notice weakness, muscle twitching, or difficulty speaking or swallowing, seeing a neurologist early is important. ALS is diagnosed through a combination of tests: electromyography (EMG), which measures electrical activity in muscles; nerve conduction studies; MRI to rule out other conditions; and sometimes a spinal tap. There is no single test that proves ALS, so diagnosis usually takes time and multiple visits.

Early diagnosis does not change the course of the disease, but it does give you time to plan, start treatment options that may slow progression, and connect with support services. The sooner you know, the sooner you can make decisions about your care and your future.

Frequently Asked Questions

Can you catch ALS from someone who has it?

No. ALS is not contagious and cannot be transmitted from one person to another through contact, saliva, or any other means. You cannot catch it from a family member, caregiver, or anyone else.

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

Not necessarily. If your parent has sporadic ALS (90 percent of cases), your risk is not higher than the general population. 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.

Can stress or injury cause ALS?

There is no evidence that stress causes ALS. Some people notice symptoms after an injury, but this is likely coincidence—the disease was already developing, and the injury drew attention to the weakness. Injury does not trigger ALS.

Is there a test that shows whether I will get ALS?

Genetic testing can show whether you carry a mutation linked to familial ALS, but it cannot predict whether you will develop symptoms. If you have a family history of ALS, a genetic counselor can help you understand what testing means and whether it is right for you.

What is the difference between ALS and other muscle diseases?

ALS specifically damages motor neurons, causing progressive weakness and muscle loss. Other muscle diseases like muscular dystrophy or myasthenia gravis affect muscles differently. A neurologist can run tests to tell the difference and confirm an ALS diagnosis.