What disease makes you very weak? Understanding the causes of severe asthenia

October 1, 2025 •

4 min read

Asthenia, the medical term for body weakness, can be a symptom of conditions ranging from mild infections to serious chronic diseases. For those experiencing severe, persistent weakness, it is critical to understand what disease makes you very weak and recognize that it warrants a thorough medical evaluation.

Quick Summary

Severe, unexplained weakness can indicate an underlying issue, such as chronic fatigue syndrome, anemia, myasthenia gravis, or multiple sclerosis, and requires professional diagnosis. The intensity, associated symptoms, and patterns of weakness can help differentiate these conditions and determine the correct course of action.

Key Points

Differentiate Weakness and Fatigue: Weakness (asthenia) is a lack of strength, while fatigue is a feeling of tiredness; chronic weakness requires investigation.
Neurological Disorders are Common Culprits: Conditions like Myalgic Encephalomyelitis/CFS, Myasthenia Gravis, and Multiple Sclerosis directly affect nerve-muscle communication, causing severe weakness.
Anemia Causes Weakness from Within: Anemia, a deficiency in red blood cells or hemoglobin, reduces oxygen delivery to tissues, leading to generalized weakness and fatigue.
Hormonal Imbalances Can be to Blame: Thyroid disorders (hypothyroidism) and adrenal gland issues (Addison's disease) can disrupt energy regulation and cause significant weakness.
Autoimmune Diseases Attack the Body: Many autoimmune conditions, including MG, MS, and rheumatoid arthritis, involve the immune system attacking healthy tissue, resulting in muscle weakness.
Infections Can Have Lasting Effects: Certain infections (e.g., flu, mono, COVID-19) can cause lingering weakness, sometimes leading to chronic conditions.

What is Asthenia? Differentiating Weakness from Fatigue

Asthenia is the medical term for a feeling of general, overall weakness or lack of energy. It is often confused with fatigue, but there are key differences. While fatigue is a state of feeling tired or weary, asthenia is a lack of strength or muscle power. Asthenia may be localized to a specific part of the body, or it can be widespread, making even simple tasks feel impossible. While temporary weakness can be caused by lifestyle factors like lack of sleep or physical exertion, persistent or severe asthenia is a sign of an underlying medical condition. The cause can range from a minor illness to a life-threatening emergency.

Neurological and Autoimmune Conditions

Several neurological and autoimmune diseases can cause extreme and debilitating weakness. These conditions often affect the communication pathways between the brain, nerves, and muscles.

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

This complex, chronic illness is defined by severe, disabling fatigue that lasts for at least six months and is not relieved by rest. The hallmark symptom is post-exertional malaise (PEM), a worsening of symptoms after even minor physical or mental activity. Other symptoms include:

Unrefreshing sleep
Cognitive issues, often called “brain fog,” which include memory and concentration problems
Dizziness or lightheadedness upon standing (orthostatic intolerance)
Muscle or joint pain
Headaches
Sore throat and tender lymph nodes

Myasthenia Gravis (MG)

Myasthenia gravis is an autoimmune disorder that interrupts nerve-muscle communication, leading to muscle weakness that fluctuates and worsens with activity. The weakness improves with rest. It often begins with eye muscle issues, such as:

Drooping eyelids (ptosis)
Double vision (diplopia)
Generalized weakness can progress to affect muscles controlling speech, swallowing, and breathing, potentially causing a life-threatening myasthenic crisis.

Multiple Sclerosis (MS)

MS is an autoimmune disease in which the immune system attacks the myelin sheath protecting nerve fibers in the central nervous system. This damage disrupts nerve signals, leading to a wide range of neurological symptoms. Common symptoms include:

Muscle weakness and stiffness, especially in the arms and legs
Numbness or tingling sensations
Fatigue
Problems with coordination and balance
Vision problems
Bladder and bowel dysfunction

Other Systemic Causes of Weakness

Anemia

Anemia is a condition characterized by a lack of healthy red blood cells or hemoglobin, which is essential for carrying oxygen throughout the body. With insufficient oxygen delivery, the body's tissues and organs cannot function properly, leading to weakness and fatigue.

Iron deficiency anemia: The most common type, resulting from insufficient iron for hemoglobin production.
Vitamin deficiency anemias: Caused by a lack of vitamin B12 or folate, which are necessary for red blood cell formation.
Other causes: Anemia can also result from chronic diseases, blood loss, or genetic factors.

Endocrine and Metabolic Disorders

Imbalances in the endocrine system can significantly impact energy levels and muscle function. Conditions such as:

Hypothyroidism: An underactive thyroid gland doesn't produce enough hormones to regulate the body's energy usage, leading to fatigue, weakness, and weight gain.
Addison's Disease: When the adrenal glands don't produce enough cortisol and aldosterone, it can cause chronic fatigue, muscle weakness, and gastrointestinal issues.
Diabetes: Poorly managed blood sugar levels can damage nerves over time and lead to weakness and fatigue.
Electrolyte Imbalances: Levels of electrolytes like potassium, calcium, and sodium that are too high or too low can disrupt nerve and muscle function.

Comparative Overview of Conditions Causing Weakness


Feature	Myalgic Encephalomyelitis/CFS	Myasthenia Gravis	Multiple Sclerosis
Type of Illness	Chronic neurological and immune disorder	Autoimmune disorder	Autoimmune disorder affecting the CNS
Nature of Weakness	Severe, unrefreshing fatigue worsened by exertion (PEM).	Fluctuating muscle weakness improving with rest.	Weakness due to nerve signal disruption, can be progressive.
Key Trigger	Physical or mental exertion, leading to post-exertional malaise.	Repeated muscle use, emotional stress, or infection.	Stress, heat, illness, and hormonal changes can trigger flare-ups.
Associated Symptoms	Brain fog, unrefreshing sleep, pain, orthostatic intolerance.	Droopy eyelids, double vision, difficulty chewing or swallowing.	Numbness, vision problems, coordination issues, fatigue, bladder problems.
Onset	Can be sudden or gradual, sometimes post-infection.	Can occur at any age, often starts with eye symptoms.	Most common between ages 20 and 40.

The Role of Infections and Other Factors

Infections are a common cause of temporary weakness. However, some infections can lead to chronic conditions, or prolonged weakness can be a symptom of ongoing health issues. This includes:

Infections: Viruses like influenza, mononucleosis (Epstein-Barr virus), or COVID-19 can cause significant weakness that may persist long after the initial illness.
Chronic Diseases: Conditions like heart failure, kidney disease, cancer, and liver disease can all contribute to severe fatigue and weakness.
Malnutrition and Deficiencies: Inadequate intake of essential nutrients like iron or vitamins can cause weakness.
Sarcopenia: The age-related loss of muscle mass and strength can lead to weakness and reduced mobility.
Certain Medications: Side effects from some antidepressants, statins, and blood pressure medication can include muscle weakness and fatigue.

Conclusion

While a minor illness or a temporary lifestyle factor can cause weakness, persistent or severe asthenia is often a sign of a more serious underlying condition. As shown, potential diagnoses range from chronic fatigue syndrome and autoimmune disorders like myasthenia gravis and multiple sclerosis to systemic problems such as anemia and endocrine disorders. Anyone experiencing prolonged or worsening weakness should seek medical advice from a healthcare professional for a proper diagnosis and appropriate management plan. Getting to the root cause is the most effective way to address symptoms and improve quality of life. For more information on Chronic Fatigue Syndrome, refer to the CDC's resources.

When to Seek Medical Attention

If you experience severe or unexplained weakness, especially if accompanied by other symptoms, it's crucial to consult a doctor. Immediate medical attention is necessary if weakness occurs alongside:

Chest pain
Shortness of breath or trouble breathing
Changes in vision
Difficulty speaking or swallowing
Severe headache
Dizziness or fainting
Irregular heartbeat

Frequently Asked Questions

Feeling weak, known as asthenia, is a lack of strength and physical power, while being tired is a feeling of weariness. If you can't perform tasks that normally require a certain amount of strength, you are experiencing weakness. If you simply feel exhausted but still have your muscle strength, you are experiencing fatigue.

Yes, nutritional deficiencies can cause significant weakness. Specifically, a lack of iron or vitamin B12 can lead to anemia, a condition where the body doesn't have enough red blood cells to carry oxygen efficiently, resulting in fatigue and weakness.

No, while ME/CFS is a prominent cause of severe weakness and post-exertional malaise, it is not the only one. Other conditions include Myasthenia Gravis, Multiple Sclerosis, anemia, thyroid disorders, and various infections.

Myasthenia gravis is an autoimmune disease where the immune system attacks the communication points between nerves and muscles. This attack prevents nerve signals from properly reaching the muscles, causing weakness that worsens with activity and improves with rest.

You should see a doctor if your weakness is severe, unexplained, or persistent. You should seek immediate medical attention if weakness is accompanied by chest pain, shortness of breath, sudden changes in vision, or difficulty speaking.

Yes. While viruses like the flu cause temporary weakness, some infections, such as Epstein-Barr virus (mononucleosis) and COVID-19, are known to lead to prolonged or chronic weakness and fatigue. Long COVID, for instance, often meets the diagnostic criteria for ME/CFS.

Yes, stress and lifestyle factors like poor sleep or an unbalanced diet can contribute to fatigue and weakness. While they may not be the sole cause of severe weakness, they can exacerbate symptoms of an underlying medical condition.