Feeling a flush, but not sure if it’s a fever? The back of your hand is a great first step (thanks, mom), but it’s really not the most reliable way to tell (sorry, mom).

First, what is a fever? It’s when your temperature rises above 100°F (38°C). Your body temperature fluctuates throughout the day, according to a research review. But the average body temp is 98.6°F (37°C).

One study showed that fever symptoms are a sign that your immune system is working properly and typically indicate the presence of a bacterial or viral infection. The heat created by your body helps fight the infection by killing off the invading microbe or virus.

While taking your temperature with a thermometer can give you a heads-up if you’re body temp is in fever territory, it’s important to note that age and how you take your temperature also plays a role (a bum temp reading will be different from under the tongue).

Here’s everything you need to know about fever symptoms, when to call a doctor, and how to feel better.

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Fever symptoms vary by age, as does treatment. A temperature range that’s no risk to an adult can be the cause for concern in a child. The average body temperature falls around 98.6°F (37°C). However, what’s normal for any one person can vary by up to .9°F (-17°C).

It’s not a bad idea to know where your normal body temperature falls because it varies for people according to a small study. If you normally run half a degree lower than normal, what’s considered a fever might be lower than average too. It will help you determine if or when to call the doctor.

Fever symptoms in adults

According to a research review, for the average adult, a fever is considered at least 100°F (38°C) oral temperature (under the tongue) and 100.4°F (38°C) rectal (ahem, your back door).

A temperature between 100°F (38°C) and 102°F (39°C) is considered standard — e.g. it’s uncomfortable, but not dangerous, and you can typically ride it out at home.

If you have an underlying condition or are immunocompromised, ask your doctor when your temperature requires a checkup or medical attention.

According to the Centers for Disease Control and Prevention (CDC), you might also experience several of the following symptoms, depending on the fever’s cause:

Fever symptoms in pregnancy

Fever symptoms during pregnancy are the same you’d experience if you weren’t pregnant. However, the threshold for safety differs because the fever’s cause of infection could be dangerous to both mom and baby. If you’re running a temperature over 101°F (38°C), call the doctor.

According to the Office of Women’s Health, in addition to the above symptoms, fever symptoms during pregnancy may also include:

These additional symptoms can often help identify the type of infection causing the fever, so take note of them and report them to your doctor.

Fever symptoms in children (3+ years old)

For children, a rise in temperature isn’t considered a fever until it reaches 100°F (38°C) oral temperature and 100.4°F (38°C) rectal temp, just like adults. In addition to the above symptoms, according to a research review, children may also show symptoms like:

  • irritability
  • behavior changes
  • difficulty waking
  • difficulty breathing
  • infrequent urination
  • burning while urinating
  • abdominal pain

Fever symptoms in babies (Under 3 years old and infants)

For children under 3 years old, a fever is generally considered 99°F (37°C) for an armpit temp, 100.4°F (38°C) rectal.

It’s important to remember that ear temperatures will not be accurate on babies younger than 6 months. Additionally, an under-the-tongue temperature may not work for younger kiddos who won’t understand how to stay still for the reading.

When in doubt, always call your pediatrician — they are there to help!

For children 3 months to 3 years old, additional fever symptoms could include:

  • poor feeding
  • problems waking up
  • feeding problems
  • decreased urine output
  • heat rash or dark rash

Infants under 3 months old may also:

  • become inconsolable
  • become lethargic
  • begin vomiting
  • have diarrhea
  • develop feeding problems
  • develop breathing problems

Bacterial and viral infections are the most common causes of fever for all age groups. Everything from the common cold, flu, and urinary tract infections to rarer conditions like fifth disease and Crohn’s disease can include a fever.

The causes are wide and varied, so try not to jump to the worst conclusion when a fever shows up. You may feel crummy, but chances are the fever will go away within a couple of days.

While fevers aren’t all bad, they can (at times) come with some complications. It all comes down to the source of the fever and the age of the person with the fever.

For example, children between the ages of 6 months and 5 years old may have febrile seizures, according to a research review. These seizures happen when the child has a sudden high fever.

They’re scary for parents and kids, but usually, they’re not dangerous and aren’t usually connected with seizures later in life. Children simply outgrow them.

The underlying cause of the fever almost always poses a greater danger than the fever itself. However, a high fever, one that’s above 103°F (39°C) for adults and lasts several days without responding to medication can cause more severe conditions. According to a research review, symptoms and complications may include:

  • changes in pupil size (nonreactivity to light)
  • losing consciousness
  • abrupt change in alertness
  • severe dizziness
  • difficulty walking
  • organ failure
  • amputation
  • brain injury
  • seizures

Call the doctor when a newborn or infant under the age of 3 months has a fever over 100.4°F (38°C). Fever means infection, and at this young age, they may not have the immune strength to fight that infection without help.

For children 3 to 6 months old, call the doctor if they have a temperature over 102°F (39°C) and show unusual lethargy, discomfort, or irritability.

For children 6 months to 3 years old, a fever of 102°F (39°C) or higher could indicate a more serious infection and warrants a call to the doctor if:

Follow the doctor’s recommendations. The doctor may request an office visit or suggest trying over-the-counter pain medications to reduce the fever or trying physical means to reduce the fever, like a tepid (not cold) bath. Schedule a visit if the fever continues for longer than 2 days.

For adults, a fever over 103°F (39°C) that lasts more than 3 days and doesn’t respond to over-the-counter medications warrants a call to the doctor.

If you have a compromised immune system or underlying medical condition, call the doctor. If you’ve been exposed to or suspect COVID-19, call your doctor and get tested.

While the list of fever complications is scary, the majority of fevers aren’t dangerous. Call your doctor if you’re concerned, based on your age, additional medical conditions, or additional symptoms. Be ready to report the temperature’s degree, duration, and accompanying symptoms.

Once you’re sick, there’s not a lot you can do to prevent a fever. However, you can take preventative measurements to prevent illness in the first place by practicing good hygiene:

  • Wash, wash, washing your hands and then, washing them again. Make sure to wash after using the bathroom, being in public, and before eating.
  • Teach children how to correctly wash their hands and watch them do it to make sure they can do it well.
  • When you don’t have access to soap and water, use hand sanitizer or antibacterial wipes. Keep some in the car, in your briefcase or purse, and at work.
  • Keep your hands away from your eyes, nose, and mouth.
  • Use your elbow to cover your mouth when you sneeze or cough. Wash your hands afterward.
  • Do not share cups, bottled drinks, or eating utensils with other people.
  • Get 7 to 9 hours of sleep every night, and eat a balanced diet.