People who are sick commonly experience chills with a fever. They may feel very cold, shiver, or shake. They might also alternate between feeling very cold and very hot.

Many other symptoms may also occur with a fever, including a more rapid heart rate, faster breathing, changes in metabolism, and increased activity in the immune system. This increased immune activity may help a person fight off an infection.

The body retains the ability to regulate temperature, so most fevers are not dangerous. That said, it is also important to note that some infections leading to a fever can be dangerous. Less commonly, a fever with chills might signal another problem, such as a reaction to a medication or cancer.

There are many viral and bacterial infections that can cause a sudden fever with chills. Keep reading to learn more about why a fever may cause chills, how and when to treat these symptoms, and when to contact a doctor.

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When a person has a fever, muscle contractions cause shaking and shivering. The purpose of these muscle contractions is to raise body temperature to help a person fight an infection or another illness.

The hypothalamus sits at the base of the brain. It is the small area responsible for temperature regulation, among other things. It is the part of the brain that establishes a healthy “set point,” or an optimum level, for a person’s temperature, which is around 37ºC (98.6ºF). When body temperature deviates from this, the body tries to return itself to the optimum temperature. It may do this by causing sweating to lower temperature or shivering to raise temperature.

When a person has a fever, the set point increases as the body tries to fight the infection. As long as a person’s body temperature is below this set point, they will feel cold. The feeling of coldness, or of being below the new optimum temperature, leads to shivering. This shivering helps raise body temperature and may also encourage a person to put on more clothing or take other measures to keep warm.

People who wish to treat a fever can try over-the-counter anti-fever medications, such as acetaminophen, aspirin, or ibuprofen.

Never give a child aspirin, as this increases the risk of a rare but life threatening condition called Reye’s syndrome.

Drinking plenty of fluids can help prevent dehydration. There is no need to take ice baths, remove clothing, or do anything else that causes discomfort. If the fever is very high and does not respond to medication, or if a person has other symptoms of severe illness — such as a stiff neck, confusion, or difficulty breathing — contact a doctor or go to the emergency room.

Most fevers, even high fevers, are not dangerous. This is because the body can still regulate its temperature and only has to set the optimum temperature higher than the usual 37ºC (98.6ºF). Treating a fever will not cure the underlying illness, and many doctors argue that treating a fever serves no medical purpose. In fact, there is little scientific evidence supporting the treatment of fevers.

One 2020 paper argues that for minor infections that are likely to go away on their own, treating a fever is probably harmless. However, for more serious infections — such as COVID-19 — treating a fever may suppress the body’s natural immune response, which could slow recovery and increase the risk of long-term complications.

There are some exceptions to this rule, however. People with weak immune systems due to having cancer, having HIV, or taking immunosuppressant drugs should contact a doctor right away for a fever. Also, in newborns younger than 3 months old, a rectal temperature higher than 38ºC (100.4ºF) or an oral temperature above 37.5ºC (99.5ºF) warrants a doctor’s visit.

Some other symptoms a person might experience along with a fever include:

  • shaking or shivering
  • feeling very cold
  • feeling hot when the fever breaks or after taking fever medication
  • symptoms of illness, such as a cough, an earache, or muscle pain
  • intense exhaustion
  • vomiting or diarrhea

Sometimes, the symptoms that accompany a fever offer a clue about its cause. For example, a person who has a fever and an earache may have an ear infection.

It is not always possible to self-diagnose, however. People who have reactions to medications that cause fevers may develop other symptoms of illness, such as muscle pain or a runny nose.

Learn more about the symptoms of a fever here.

It is important to contact a doctor if:

  • A person develops any symptoms of a serious illness, such as meningitis. These symptoms may include a stiff neck, confusion, or sensitivity to light.
  • Symptoms of a milder illness continue to worsen or do not go away after a few days.
  • A newborn under 3 months old has a fever.
  • A person with a serious illness or a weak immune system gets a fever.
  • A person has chronic, unexplained fevers.
  • A person develops a fever after taking a new medication.

Often, the best strategy for preventing fevers is to prevent the infections that cause them. The following tips may help:

  • Stay home when sick, and do not send sick children to daycare or school.
  • Know that treating a fever does not prevent it from spreading. If a person still has a fever despite taking medications, they may still be contagious.
  • Get all recommended vaccines.
  • Practice regular hand-washing with soap and warm water, especially before eating, before touching the face, and after touching someone else.
  • Try to avoid people who are sick. If doing so is impossible, wear a mask and frequently wash the hands.
  • Practice safer sex to reduce the spread of sexually transmitted infections.
  • In crowded areas where COVID-19 rates are high, wear a mask indoors.
  • Practice safe food preparation, including using different utensils for different foods, heating food according to the manufacturer’s instructions, and washing the hands before and after touching raw meat.

A fever will often present with chills, as it is the body’s reaction to trying to raise its core temperature to an optimum level when fighting certain infections and viruses.

A fever can be very unpleasant, and it may cause painful symptoms — even when the cause is a relatively minor illness. However, a fever is the body’s normal reaction to an infection or illness, and it is not a reason to panic. Although the infections that cause fevers can be dangerous, fevers themselves are rarely dangerous.

People should aim to focus on self-care and comfort strategies, such as getting warm, resting, and drinking plenty of fluids. Taking a fever medication may ease the symptoms and help a person rest better.

Most fevers will go away on their own. However, if the accompanying symptoms are very severe, get worse, or do not resolve within a few days, contact a doctor.