Why Do We Feel Cold When We Have a Fever?

We've all been there: burning with fever — and shivering with chills at the same time. It turns out that what feels like an odd internal thermostat malfunction is actually a sign that your body is fighting off an infection.

The first thing to understand is that most viruses and bacteria have a hard time surviving above normal human body temperature, which can vary by age, activity, and time of day, but is generally accepted as 98.6 degrees F, according to the U.S. National Library of Medicine.

In fact, even just a one- or two-degree hike in temperature can stop many invading microorganisms in their tracks. So it’s no wonder that over millions of years, fever evolved as a means of helping the body defend itself. “Part of our immune system's response against infection includes raising the body's temperature to diminish the ability of microbes to reproduce," explains Amesh Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security in Baltimore.

What Makes You Feel Colder When Your Body Is Hotter?

It’s actually a normal physiological response. As soon as your brain shifts its internal thermostat to a higher set point to fight off an infection, the rest of your body goes to work trying to generate extra heat to meet that higher temperature goal. Suddenly, you’re technically below your new “ideal” core temperature, so you feel cold.

Feeling chilled then prompts you to start shivering and even shaking “as your body tries to generate heat to raise your temperature by making your muscles contract,” explains Nate Favini, MD, medical director of the nationwide healthcare system Forward.

How Long Do Fever and Chills Last in Adults?

The length of a fever — and any accompanying chills — can vary significantly depending on its cause. "In some cases with a mild viral illness, a fever can last for a day, or it can last for weeks to months with systemic infections," says Dr. Adalja.

The best thing to do is play detective to determine the source of your fever based on other signs and symptoms of your illness, he says. Possible causes abound, including colds and the flu, bronchitis, pneumonia, appendicitis, gastroenteritis, mononucleosis, ear infections, sinus infections, and urinary tract infections (UTIs).

While fevers are usually caused by viruses, other conditions that can spike your temperature include certain inflammatory disorders such as lupus, rheumatoid arthritis, and Crohn's disease, as well as cancer and blood clots (deep vein thrombosis). Even certain medication such as penicillin, sulfa drugs, and antipsychotics can trigger a fever, as can some illegal drugs such as cocaine, according to the Merck Manual.

What Should You Do When You Have the Chills and Fever?

Fevers in adults and the chills that go hand in hand generally resolve within a few days, note Mayo Clinic experts. If your temperature is mildly elevated — between 100 degrees F and 102 degrees F — and you have no other worrisome symptoms (see below), simply rest and drink plenty of fluids to keep from becoming dehydrated.

However, even though you feel cold on the outside, you should keep clothing and blankets light to prevent overheating because inside your body is very hot, say experts at UPMC/University of Pittsburgh Schools of the Health Sciences.

When Should I Call the Doctor?

If your temperature hits 103 degrees F, Mayo Clinic experts advise you to contact your doctor. Per the Merck Manual, you should also consult him or her if:

• You have a serious medical condition that could make a moderate fever more dangerous, such as a heart or lung disorder
• A fever lasts more than 24–48 hours

Seek medical help immediately if anything unusual or alarming accompanies the fever and chills, such as any of these symptoms:

• A change in mental function, such as confusion
• A headache, stiff neck, or both
• Flat, small, purplish red spots on the skin, which indicate bleeding under the skin
• Low blood pressure
• Rapid heart rate or rapid breathing
• Shortness of breath
• Recent travel to an area where a serious infectious disease such as malaria is common
• Recent use of drugs that suppress the immune system (immunosuppressants)

What Medication, if Any, Should I Take?

Because fever helps the body defend against infection and because a moderate fever itself is not dangerous, Mayo Clinic pros say it’s often best for otherwise healthy adults to let one run its course.

If you’re uncomfortable, however, consider taking an over-the-counter fever reducer, such as Tylenol (acetaminophen), a nonsteroidal anti-inflammatory drug (NSAID) such as aspirin, ibuprofen (Advil, Motrin), or Aleve (naproxen). Follow instructions on the label precisely for proper dosage.

Also be careful not to take more than one medication containing acetaminophen, which is a common ingredient in many over-the-counter medications, including cough and cold remedies. Acetaminophen can cause serious harm to the liver if you take too much.