What illnesses can cause weakness? A comprehensive overview

September 26, 2025 •

5 min read

According to the American Academy of Family Physicians, muscle weakness in adults aged 60 and older is a common symptom, though often difficult to evaluate. The reasons behind this symptom are incredibly diverse, prompting many to ask: What illnesses can cause weakness? Understanding the potential causes, from minor issues to serious conditions, is the first step toward finding a resolution.

Quick Summary

Illnesses causing weakness range from temporary infections like the flu to chronic conditions such as diabetes, thyroid disorders, and neurological diseases like multiple sclerosis. Nutritional deficiencies, autoimmune disorders, and medication side effects can also be contributing factors.

Key Points

Differentiate weakness and fatigue: True muscle weakness, or asthenia, is a loss of strength, while fatigue is a feeling of tiredness. They often coexist but have distinct meanings.
Infections are a common cause: Viral infections like the flu, COVID-19, and mononucleosis can cause both short-term and prolonged weakness as the body fights off the infection.
Neurological issues are a serious concern: Conditions like Multiple Sclerosis, Myasthenia Gravis, and stroke directly affect nerve and muscle communication, leading to localized or generalized weakness.
Metabolic and hormonal imbalances play a role: Thyroid disorders (hyper- and hypothyroidism) and diabetes can disrupt the body's energy regulation, resulting in muscle weakness and fatigue.
Nutrient deficiencies are often overlooked: Low levels of vitamins D and B12, as well as minerals like iron and potassium, can impair nerve and muscle function.
Autoimmune disorders cause inflammation: Diseases such as polymyositis and rheumatoid arthritis involve the immune system attacking healthy tissue, leading to inflammatory myopathies and weakness.
Seek immediate care for sudden weakness: If weakness comes on quickly, affects one side of the body, or is accompanied by neurological symptoms, it could signal a medical emergency like a stroke.

Understanding the difference between weakness and fatigue

Before delving into specific illnesses, it's crucial to distinguish between true muscle weakness and generalized fatigue. Muscle weakness, or asthenia, is the inability to exert normal force with your muscles, making everyday tasks difficult. Fatigue, on the other hand, is a general feeling of tiredness or exhaustion that isn't necessarily related to a lack of muscle strength. While many conditions cause both, understanding the nuance can help pinpoint the root cause.

Common infections and post-viral syndrome

Feeling weak and exhausted is a normal part of the body's immune response to fighting off an infection. The body diverts energy to the immune system, leaving less for muscle function. However, in some cases, weakness can linger long after the initial illness has passed.

Influenza and common cold: It's common to feel weak and tired during and for a couple of weeks after a viral illness as your body recovers.
COVID-19: Many individuals experience prolonged fatigue and muscle weakness, often referred to as "long COVID," affecting their ability to perform daily activities.
Mononucleosis (Epstein-Barr virus): This viral infection is notorious for causing extreme fatigue and weakness that can last for weeks or even months.
Lyme disease: A tick-borne illness that can cause a range of symptoms, including muscle weakness, especially if not treated early.

Neurological and neuromuscular disorders

The nervous system is responsible for controlling muscle function. When nerves or the connections between nerves and muscles are damaged, weakness is a primary symptom. These can range from sudden and serious conditions to chronic, progressive diseases.

Multiple Sclerosis (MS): An autoimmune disease where the immune system attacks the protective sheath (myelin) covering nerve fibers. This disrupts communication between the brain and body, leading to numbness or weakness in one or more limbs.
Myasthenia Gravis: An autoimmune condition causing muscle weakness that worsens with activity and improves with rest. It is caused by a breakdown in communication between nerves and muscles.
Guillain-Barré Syndrome: A rare and serious disorder in which the immune system attacks healthy nerve cells in the peripheral nervous system. It can cause muscle weakness and paralysis that progresses rapidly.
Stroke: Occurs when a blood vessel in the brain is blocked or ruptures. Sudden weakness or numbness, often on one side of the body, is a hallmark symptom.

Endocrine and metabolic conditions

The body's hormonal and metabolic balance is vital for energy production and muscle function. Disruptions in this balance can manifest as widespread weakness.

Thyroid Disorders: Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can cause muscle weakness. Hypothyroidism can lead to myopathy, characterized by muscle cramps and weakness, while hyperthyroidism can cause thyrotoxic myopathy.
Diabetes: When blood sugar levels are poorly controlled, nerve damage (diabetic neuropathy) can occur, causing weakness and tingling in the hands and feet.
Adrenal Insufficiency (Addison's disease): A rare disorder where the adrenal glands don't produce enough hormones. Generalized weakness, along with fatigue and weight loss, is a common symptom.

Nutritional deficiencies

Proper nutrition is essential for muscle health and overall energy. Deficiencies in certain vitamins and minerals can impair nerve and muscle function.

Vitamin D Deficiency: Essential for calcium absorption and bone health, a lack of vitamin D can lead to muscle weakness and bone pain.
Vitamin B12 Deficiency: Crucial for nerve function and red blood cell production, low B12 levels can cause nerve damage, tingling, and muscle weakness.
Anemia (Iron Deficiency): Low red blood cell counts reduce oxygen transport to tissues, leading to generalized fatigue and weakness.
Electrolyte Imbalances: Low levels of minerals like potassium or sodium can disrupt nerve and muscle signaling, causing weakness.

Autoimmune and inflammatory diseases

These conditions involve the immune system mistakenly attacking the body's own tissues, including muscles and joints, causing inflammation and weakness.

Polymyositis and Dermatomyositis: Rare chronic inflammatory diseases that cause muscle weakness, especially in the neck, shoulders, and hips. Dermatomyositis also includes a characteristic skin rash.
Rheumatoid Arthritis: An autoimmune disorder causing joint inflammation, which can lead to fatigue and generalized weakness.
Fibromyalgia: Causes widespread chronic pain and fatigue, often accompanied by muscle weakness and stiffness.

Comparing causes of weakness


Cause Category	Common Examples	Onset Speed	Affected Muscles	Other Common Symptoms
Infections	Flu, COVID-19, Mononucleosis	Acute (sudden) to Subacute	Generalized	Fever, body aches, cough, sore throat, fatigue
Neurological	Stroke, MS, Guillain-Barré	Acute (Stroke) to Gradual (MS)	Focal (one side) to Widespread	Numbness, tingling, vision changes, coordination problems, paralysis
Endocrine	Hypothyroidism, Diabetes	Gradual	Proximal (shoulders, hips)	Weight changes, fatigue, skin changes, increased thirst/urination
Nutritional	Anemia, Vitamin D/B12 deficiency	Gradual	Generalized or Peripheral	Fatigue, pale skin, tingling, bone pain, nerve issues
Inflammatory	Polymyositis, Rheumatoid Arthritis	Gradual	Proximal (shoulders, hips)	Joint pain/swelling, rash, swallowing difficulty, fatigue

When to see a doctor

It is important to seek medical evaluation if you experience persistent or unexplained weakness. While mild, temporary weakness after a long hike or a viral illness is normal, certain signs should prompt immediate medical attention.

Sudden onset of weakness: Especially if it occurs on one side of the body, as this could indicate a stroke.
Rapidly worsening weakness: If weakness spreads quickly or becomes more severe over hours or days.
Accompanying symptoms: Seek emergency care if weakness is paired with difficulty breathing, swallowing, talking, or if it is accompanied by confusion, vision changes, or a severe headache.
Weakness lasting more than two weeks: If weakness and fatigue don't resolve with rest or seem unrelated to a minor illness.

A healthcare provider can perform a comprehensive evaluation to determine the underlying cause. This may include a physical examination, medical history review, blood tests, and specialized imaging or nerve studies. For more information on nerve and muscle disorders, a valuable resource is the National Institute of Neurological Disorders and Stroke.

Conclusion

Weakness can be a symptom of a wide range of illnesses, from short-term infections to complex, chronic conditions affecting the nervous system, hormones, or immune response. It is the body's signal that something is amiss, and paying attention to the pattern, location, and accompanying symptoms can provide crucial clues. While some causes are easily remedied with rest or dietary changes, others require prompt medical evaluation. Always prioritize consulting with a healthcare professional to get an accurate diagnosis and appropriate treatment plan, especially when weakness is sudden, severe, or persistent.

Frequently Asked Questions

Yes, deficiencies in several key vitamins can cause muscle weakness. A lack of vitamin D can impair calcium absorption, affecting bone and muscle health. Similarly, a deficiency in vitamin B12 can damage nerves and lead to weakness, tingling, and balance issues.

While fatigue is present in a majority of cancer patients, it is most often accompanied by other clinical signs of malignancy and is a rare cause of fatigue on its own in primary care. If weakness is unexplained and persistent, a doctor can perform tests to rule out serious conditions, including cancer.

The thyroid gland controls metabolism. An underactive thyroid (hypothyroidism) slows metabolism, causing a lack of energy and muscle weakness. An overactive thyroid (hyperthyroidism) can also lead to a type of muscle weakness called thyrotoxic myopathy.

Yes, it is common to experience lingering fatigue and weakness for several weeks after a viral infection like the flu or COVID-19. This is often due to the body recovering from the intense energy expenditure of fighting the virus.

You should see a doctor if weakness persists for more than two weeks or if it is unexplained. Immediate medical attention is needed for sudden onset weakness, especially if it affects one side of the body, or is accompanied by difficulty breathing, talking, or swallowing.

Autoimmune diseases, such as polymyositis and rheumatoid arthritis, cause the immune system to attack healthy tissue, including muscles and joints. The resulting inflammation and damage to muscle fibers or surrounding tissues can lead to chronic or progressive muscle weakness.

Yes, although they often occur together. Weakness is a lack of physical or muscle strength, requiring extra effort to move. Fatigue is a general feeling of tiredness or weariness. An underlying medical condition can cause either or both.