What illness makes the body weak? Understanding chronic fatigue and other causes

September 26, 2025 •

4 min read

According to the Centers for Disease Control and Prevention, an estimated 3 million people in the U.S. have anemia, a condition known to cause extreme fatigue and weakness. So, what illness makes the body weak? The answer isn't simple, as many conditions can lead to physical weakness, ranging from temporary infections to chronic diseases.

Quick Summary

Many illnesses, both short-term and chronic, can cause body weakness, including infections like the flu, systemic conditions like anemia, and autoimmune disorders such as multiple sclerosis. Determining the specific cause requires a medical evaluation, as symptoms can range from mild fatigue to severe muscle weakness.

Key Points

Infections Cause Temporary Weakness: Common illnesses like the flu and COVID-19 can cause temporary but significant weakness as your body fights the virus.
Chronic Conditions Lead to Persistent Weakness: Long-term illnesses like chronic fatigue syndrome (ME/CFS), fibromyalgia, and multiple sclerosis are known for causing ongoing weakness that is not easily resolved by rest.
Anemia Affects Oxygen Transport: Anemia, a lack of healthy red blood cells, leads to insufficient oxygen delivery to tissues, resulting in fatigue and weakness.
Neuromuscular Disorders Impact Muscle Control: Conditions such as muscular dystrophy and myasthenia gravis directly affect the communication between nerves and muscles, causing progressive weakness.
Thyroid Issues Disrupt Metabolism: Both an underactive and overactive thyroid can interfere with the body's energy use, resulting in weakness and fatigue.
Medications and Lifestyle Factors Can Contribute: Certain medications, a sedentary lifestyle, and poor sleep can all cause or worsen feelings of weakness.

Understanding the difference between fatigue and true muscle weakness

Before diving into specific illnesses, it's important to distinguish between fatigue and genuine muscle weakness. While the two often occur together, they have different root causes. Fatigue is a general feeling of tiredness or exhaustion that isn't relieved by rest. True muscle weakness, or asthenia, is a lack of physical strength, meaning the muscles can't contract normally and physical function is impaired. A single illness can present with one or both of these symptoms.

Infectious diseases and viral myositis

Fighting off an infection is a taxing process that puts a significant strain on the body's resources, often leading to temporary weakness and fatigue. The flu, COVID-19, and other viral illnesses are notorious for causing these symptoms. In some cases, a viral infection can trigger viral myositis, an inflammation of the muscle tissue itself, leading to more pronounced weakness.

Influenza (Flu): One of the most common causes of temporary weakness and myalgia (muscle aches).
COVID-19: Can cause severe fatigue and muscle aches that may persist for weeks or months, a condition known as long COVID.
Epstein-Barr Virus (Mononucleosis): Often causes profound fatigue and weakness that can last for an extended period.

Blood disorders like anemia

Anemia is a condition characterized by a deficiency of red blood cells or hemoglobin, which are crucial for transporting oxygen throughout the body. Without enough oxygen reaching the muscles and tissues, individuals can experience significant weakness and tiredness.

Iron-Deficiency Anemia: The most common type, often caused by blood loss (e.g., heavy menstruation) or inadequate iron intake.
Vitamin B12-Deficiency Anemia: A lack of this essential vitamin can disrupt red blood cell production, leading to fatigue.

Chronic conditions affecting energy and muscle function

Some chronic illnesses cause long-term, persistent weakness that doesn't resolve with rest.

Chronic Fatigue Syndrome (ME/CFS)

This complex and debilitating disorder is defined by extreme fatigue that worsens with physical or mental exertion and doesn't improve with rest. Weakness is a hallmark symptom, often accompanied by unrefreshing sleep, cognitive problems, and pain.

Fibromyalgia

Characterized by widespread musculoskeletal pain, sleep problems, and fatigue, fibromyalgia also frequently causes chronic muscle weakness. The pain and lack of restorative sleep contribute significantly to the feelings of weakness.

Thyroid disorders

The thyroid gland plays a vital role in regulating the body's metabolism. Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can cause weakness and fatigue.

Diabetes

Poorly managed blood sugar levels in diabetes can lead to nerve damage (neuropathy), which can cause muscle weakness, especially in the legs and feet.

Autoimmune diseases

These conditions cause the body's immune system to mistakenly attack healthy tissue, leading to inflammation and damage that can result in weakness.

Multiple Sclerosis (MS): The immune system attacks the protective sheath covering nerve fibers, disrupting communication between the brain and the rest of the body and causing widespread weakness.
Rheumatoid Arthritis (RA): This chronic inflammatory disorder can cause joint pain, fatigue, and muscle weakness.

Neuromuscular disorders

These conditions directly affect the nerves and muscles, causing progressive and often severe weakness.

Muscular Dystrophy: A group of genetic diseases that cause progressive muscle weakness and loss of muscle mass over time.
Myasthenia Gravis: An autoimmune disease where antibodies block communication between nerves and muscles, resulting in weakness that worsens with activity.

Comparison of different causes of body weakness


Feature	Infectious Diseases	Anemia	Chronic Fatigue Syndrome (ME/CFS)	Neuromuscular Disorders
Onset	Often sudden, associated with fever and other acute symptoms.	Gradual, symptoms may worsen over time.	Often triggered by an infection, but symptoms persist long-term.	Progressive, symptoms gradually worsen.
Symptom Duration	Temporary; resolves after infection clears.	Chronic if underlying cause isn't addressed.	Chronic; lasts for at least six months.	Chronic and progressive.
Primary Weakness Type	General fatigue, muscle aches (myalgia).	Fatigue due to lack of oxygen, overall weakness.	Extreme, debilitating fatigue and post-exertional malaise.	True muscle weakness, affecting muscle control.
Typical Associated Symptoms	Fever, cough, sore throat, body aches.	Pale skin, dizziness, shortness of breath, headache.	Unrefreshing sleep, cognitive issues, pain, dizziness on standing.	Muscle stiffness, cramps, loss of muscle mass.
Diagnosis Method	Symptom evaluation, sometimes lab tests (e.g., viral testing).	Complete blood count (CBC) and other blood tests.	Based on specific diagnostic criteria and ruling out other conditions.	Neurological exam, specialized nerve and muscle tests.

Lifestyle and other factors

Beyond specific illnesses, several other factors can contribute to feelings of weakness.

Poor sleep

Insufficient or unrefreshing sleep hinders the body's ability to repair and restore itself, leading to daytime weakness and fatigue.

Nutrient deficiencies

Deficiencies in essential vitamins and minerals, such as vitamin D and B vitamins, can severely impact energy levels and muscle function.

Sedentary lifestyle

Extended periods of inactivity, such as bed rest or a desk-bound lifestyle, cause muscles to weaken and atrophy over time.

Medications

Some prescription drugs, including statins and certain antidepressants, can cause muscle weakness and fatigue as a side effect.

When to see a doctor

If you experience sudden, unexplained, or worsening weakness, it's crucial to consult a healthcare professional. A doctor can perform a physical examination and order diagnostic tests to determine the underlying cause and develop an appropriate treatment plan. For authoritative medical information, you can always consult a reliable resource like the National Institutes of Health (NIH). https://www.nih.gov/

Conclusion: Taking a comprehensive approach

Weakness is a common symptom with a vast number of potential causes, from minor infections to serious chronic conditions. Pinpointing the exact reason requires careful evaluation by a medical professional. If you are experiencing persistent or severe weakness, pay attention to other accompanying symptoms and seek professional medical advice to ensure an accurate diagnosis and effective treatment. Managing the underlying illness, whether through medication, lifestyle changes, or other therapies, is the key to regaining strength and vitality.

Frequently Asked Questions

Yes, chronic stress and anxiety can lead to both mental and physical exhaustion. The body's constant state of high alert can deplete energy resources, leading to feelings of profound weakness and fatigue over time.

Not necessarily. Sudden weakness can be caused by minor issues like a cold or temporary dehydration. However, it can also be a sign of a serious condition like a stroke. If sudden weakness is accompanied by other severe symptoms, such as slurred speech or vision changes, seek immediate medical attention.

Anemia-related weakness is often accompanied by other symptoms, including pale skin, dizziness, shortness of breath, and cold hands and feet. A simple blood test, like a Complete Blood Count (CBC), can diagnose anemia by checking your red blood cell and hemoglobin levels.

Post-exertional malaise (PEM) is a hallmark symptom of ME/CFS where even minor physical, mental, or emotional exertion leads to a worsening of symptoms, including profound weakness and fatigue. This often occurs 12 to 48 hours after the activity and can last for days or weeks.

Yes, deficiencies in essential vitamins, particularly B12 and Vitamin D, can directly impact energy production and muscle function, leading to weakness and fatigue. A balanced diet and, if necessary, doctor-recommended supplements can help address this.

A sedentary lifestyle leads to muscle atrophy, the weakening and wasting away of muscle tissue due to lack of use. This can make everyday activities feel more difficult and cause overall body weakness. Regular, gentle activity can help prevent this decline.

You should go to the ER if weakness comes on suddenly and is severe, especially if it's accompanied by symptoms like numbness on one side of the body, difficulty speaking, vision changes, chest pain, or trouble breathing. These can be signs of a medical emergency, such as a stroke.