Flu-like symptoms are a group of symptoms that often occur together during viral infections, such as influenza (flu). The symptoms can include fever, chills, headache, and body aches.

Colds, COVID-19, and bacterial infections can also cause flu-like symptoms. However, noninfectious conditions such as autoimmune disorders, as well as certain drugs and medical treatments, can lead to flu-like symptoms.

Read on to learn more about what flu-like symptoms are, their potential causes, and when to contact a doctor.

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Doctors use the term “flu-like symptoms” as shorthand for a collection of symptoms that often occur as a result of infectious illnesses, such as flu. However, many conditions can potentially cause them.

These symptoms can include:

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Some of the most common causes for flu-like symptoms include:

Flu

Flu causes millions of people to become unwell every year. Between October 2023 and April 2024, the Centers for Disease Control and Prevention (CDC)Trusted Source estimated that there were between 34 and 75 million cases in the United States.

Flu typicallyTrusted Source develops suddenly and tends to be more severe than a cold.

Colds

Colds can also cause some flu-like symptoms, although this will rarelyTrusted Source include fever or chills. If the cause is a cold, the onset will usually be more gradual than with flu, and the symptoms less severe.

COVID-19

The symptoms of COVID-19 can be so similar to flu that they are difficult to tell apart. However, COVID-19 can cause more severeTrusted Source illness in some people than flu. It also spreads more easily and is contagious for longer.

People who could have COVID-19 should stay at home and seek testing to confirm the diagnosis.

Other viral illnesses

Because flu-like symptoms can be a result of an immune response, many other viral infections can cause them, such as:

Bacterial infections

Many bacterial infections can also cause flu-like symptoms, including:

Parasitic and fungal infections

Flu-like symptoms can also be due to infections from parasites or fungi, such as:

Below are some of the causes of flu-like symptoms that are not the result of an infection.

Stress and anxiety

Stress and anxiety can cause many of the same symptoms as flu, including:

  • sweating or feeling cold
  • tense or aching muscles
  • headaches
  • tiredness
  • nausea

Chronic stress also reduces the immune system’s defenses against infections, including flu and the common cold, according to a 2019 review. It may combine with other factors, such as circulating infections, to make viral illness more likely.

Learn more about how stress affects the body.

Autoimmune diseases

Some autoimmune disorders cause generalized symptoms that affect the whole body, such as a low grade fever, fatigue, and muscle or joint pain. Some examples of these conditions include lupus and rheumatoid arthritis.

As with infections, this can be due to activation of the immune system.

Other chronic illnesses and postexertional malaise

Several chronic illnesses can cause flu-like symptoms, such as:

For some people with these conditions, flu-like symptoms may be regular. For others, they may only occur after a person tries to do more physical activity. This is known as postexertional malaise.

Medical treatments

Several medications and treatments can cause flu-like symptoms as a side effect, including:

  • Antimicrobials: Medications that kill microbes, such as antibiotics, can result in a Jarisch-Herxheimer reaction. This occurs when large amounts of bacteria die, such as those that cause Lyme disease and syphilis, and usually sets in within 24 hoursTrusted Source of starting treatment.
  • Cancer treatments: Some types of chemotherapy and immunotherapy can cause flu-like symptoms. This usually happens shortly afterTrusted Source treatment and can range from mild to severe.
  • Vaccines: ManyTrusted Source vaccines, such as the flu and COVID-19 vaccines, can cause temporary flu-like symptoms.

Drug withdrawal

Some drugs cause withdrawal when a person reduces their dosage or stops taking them. This can happen with both legal and illegal or prescription-only drugs, such asTrusted Source:

The symptoms of withdrawal can vary between drugs and individuals, but can include flu-like symptoms such as fatigue, nausea, body aches, headaches, and feeling cold.

For some drugs, such as alcohol and benzodiazpines, withdrawal can be dangerous. Always seek guidance from a doctor before stopping or changing the dose of a drug.

Can you get flu-like symptoms after donating blood?

Some people feel dizzy, sweaty, or nauseous during or after having a blood draw. This can be a vasovagal response, which is a nervous system reaction that makes a person feel faint.

People experiencing these symptoms should lie down right away, drink fluids, and rest until they feel better. However, if these symptoms do not get better with rest, this may indicate a person had an infection when they gave blood.

Notify the doctor or blood donation center if this occurs.

Can dental work cause flu-like symptoms?

Some people experience flu-like symptoms after dental work. This may be due to complications, such as a negative reaction to anesthesia, an allergic reaction, or an infection.

Stress or anxiety during the procedure could also cause some flu-like symptoms afterwards.

People who feel unwell after dental work should talk to their dentist right away, particularly if they have a fever, swelling, and pus or a bad taste in the mouth.

Anyone with a fever, chills, or other flu-like symptoms can contact a doctor for advice. Unless it is an emergency, a person should call first rather than visiting in-person if it is possible they may have a contagious illness.

Get in touch with a doctor right away if:

  • a child has a fever
  • a person has had an animal bite, even if there are no other symptoms
  • a person has had exposure to an STI or blood borne illness, such as hepatitis or HIV
  • flu-like symptoms develop in the following situations:
    • after a tick bite
    • after a dental, cosmetic, or surgical procedure
    • after visiting a place where potentially serious infections, such as malaria, are common
    • after starting, reducing, or stopping a drug
  • flu-like symptoms get better but then come back or worsen

Seek urgent medical care if a person develops:

Flu-like symptoms can include fatigue, a sore throat, headache, body aches, fever or chills, and a general feeling of being unwell. Flu is a potential cause for this group of symptoms, but many other infections can cause them, too.

Sometimes, flu-like symptoms can occur for other reasons, such as stress, drugs, medical treatments, vaccines, or noninfectious chronic conditions.

Because many conditions can cause flu-like symptoms, it is important to speak with a doctor if they persist, are severe, or could be a sign of a serious illness.