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It’s normal for your body temperature to fluctuate throughout the day. But in general, if you’re an adult and your temperature is above 100.4°F (38°C), you have a fever.

A fever is the body’s way of battling an illness. Although it’s possible to have one without a known cause, fevers are usually brought on by a virus or bacterial infection.

Before you start searching for a thermometer, though, take stock of your symptoms. Are you clammy? Tired? The symptoms of a fever can get even more tricky in infants and toddlers.

The most common symptoms of fever include:

  • headache
  • warm forehead
  • chills
  • aching muscles
  • general feeling of weakness
  • sore eyes
  • loss of appetite
  • dehydration
  • swollen lymph nodes

Infants or young children who have a fever may also experience:

  • greater irritability than usual
  • lethargy
  • flushed skin
  • paleness
  • difficulty swallowing
  • refusal to eat, drink, or breastfeed

In severe cases, a fever may cause:

  • excessive sleepiness
  • confusion
  • convulsions
  • severe pain in other parts of the body
  • unusual vaginal discharge
  • pain during urination
  • skin rash
  • vomiting
  • diarrhea

Keep reading to learn the different ways to check your temperature, plus tips on how to bring a fever down and more.

Fever and COVID-19

In early 2020, a new virus started making headlines for causing the disease known as COVID-19. One of the telltale symptoms of COVID-19 is a low-grade fever that gradually worsens over time.

Other common symptoms of COVID-19 include shortness of breath and a dry cough that gradually becomes more severe.

For most people, these symptoms will resolve on their own, and no medical attention is needed. However, you should contact emergency services if you experience difficulty breathing, confusion, blue lips, or persistent chest pain.

There are several ways to take your temperature. Each has its pros and cons.

Mouth

Oral thermometers are used to take a temperature in the mouth. They usually have a digital readout, beep when the reading is complete, and may even alert you if the temperature is high enough to be considered a fever.

Taking a temperature by mouth is a better option for adults than for children and babies. That’s because to get an accurate reading, you need to keep your mouth closed with the thermometer held in place for at least 20 seconds. This can be difficult for children and babies to do.

To use an oral thermometer:

  1. Avoid eating or drinking 15 minutes prior to inserting the thermometer. That’s because food and drinks can alter the temperature in your mouth and affect the reading.
  2. Hold the thermometer underneath your tongue for at least 20 seconds before removing it. It should be as close to the center of your mouth as possible. This may vary based on brand, so be sure to check the instructions for your specific thermometer.
  3. After you get a reading, disinfect the thermometer with antibacterial soap and warm water.

Ear

Ear-based thermometers measure the temperature of the tympanic membrane. This is known as the eardrum. Although medical professionals often use them, you can use an ear-based thermometer at home too.

An ear-based thermometer uses a digital readout and delivers results in seconds. Babies older than 6 months, children, and adults can use one. Because it’s fast, it’s often an easy option for parents to use on young children.

A 2013 study found that this type of thermometer is as effective as a mercury-in-glass thermometer.

To use a digital ear thermometer:

  1. Hold the thermometer up to your ear, with the infrared sensor pointing toward your ear canal.
  2. When the thermometer is in place, turn it on. Most models will beep when the reading is complete.

Don’t insert an ear thermometer into the ear canal. Because it uses infrared radiation, the thermometer can get a reading if the sensor is pointing toward the ear canal.

Rectal

You can get a rectal temperature by gently inserting a thermometer into your rectum. You can use a standard thermometer — same as what you’d use to take your temperature by mouth. But you should never use the same thermometer in your mouth that you’ve used in your rectum.

Instead, purchase two thermometers and label each one for how it’s used. You can also purchase online a rectal thermometer with a small tip to use for a baby. It can reduce the risk of injuring your baby.

A 2015 study found that a rectal temperature reading is more accurate than an oral or ear-based one.

Rectal thermometers are the best choice for young children, especially those under the age of 6 months. That’s because you’ll be able to get a more accurate reading. In fact, many pediatricians will request you take a rectal temperature before seeing them for a fever in a baby.

To take your baby’s rectal temperature:

  1. Turn your baby onto their stomach and remove their diaper.
  2. Gently insert the thermometer tip into the rectum. Don’t insert it more than 1/2 inch to 1 inch.
  3. Turn the thermometer on and hold it in place for about 20 seconds.
  4. When the reading is complete, gently remove the thermometer.
  5. Clean the rectal thermometer with rubbing alcohol after use.

You may also want to consider using disposable thermometer sleeves, especially if you’ll be using the thermometer on more than one person.

If your baby moves around a lot during the reading, the results may be inaccurate.

Without a thermometer

If you don’t have a thermometer, there are less accurate ways you can diagnose a fever.

Touch is the most popular method, but it’s also the least accurate. This is especially the case if you’re self-diagnosing.

When using touch to diagnose a fever in someone else, touch your own skin first, then touch the other person to compare the two temperatures. If the other person is a lot hotter than you, they may have a fever.

You can also try pinching the skin on the back of your hand to check for signs of dehydration. If the skin doesn’t snap back quickly, you could be dehydrated. Dehydration may be a sign of a fever.

You have a fever if your rectal temperature is 100.4°F (38°C) or your oral temperature is 100°F (37.8°C). In adults and children over 3 months, a temperature of 102.2°F (39°C) or higher is considered a high fever.

If your baby is up to 3 months old and has a rectal temperature of 100.4°F (38°C), seek immediate medical help. Fevers in young babies can be very serious.

If your child is between 3 months and 3 years old and has a temperature of 102.2°F (39°C), call their doctor. This is considered a high fever.

In anyone, a temperature of more than 104°F (40°C) or less than 95°F (35°C) is cause for concern. Seek immediate medical help if this is the case.

Unless your fever is the result of an underlying illness, such as an infection, or the fever is in a young baby or child, medical attention usually isn’t necessary.

Tips for treating a fever

  • Avoid the heat. If you can, keep the room temperature cool. Swap out thicker materials for light, breathable fabrics. Opt for a sheet or light blanket at night.
  • Stay hydrated. Replenishing lost fluids is key. Water is always a good option, but broth or a rehydration mix like Pedialyte can also be beneficial.
  • Take a fever reducer. Fever-reducing medications like ibuprofen (Advil) and acetaminophen (Tylenol) can also help alleviate symptoms. Talk to your doctor before offering these medications to a baby or child to get the OK and correct dose.
  • Rest. Activity can raise your body temperature, so take things slow while you wait for the fever to pass.
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Should you take a cold bath or shower?

Cold water can temporarily help reduce your temperature, but it can lead to shivering.

When you shiver, your body rapidly vibrates to increase your body temperature, so you could actually cause your temperature to get higher if you take a cold bath or shower.

Instead, try sponging your body with warm water. As the water evaporates, your body will begin to cool. If sponging causes shivering, though, stop or increase the water temperature.

In most cases, fevers will run their course.

However, there are instances when medical attention is necessary in adults. If your temperature exceeds 104°F (40°C) or it’s not responding to fever-reducing medication, consider contacting your doctor.

For babies 3 months and younger, seek immediate medical help if they have a rectal temperature of 100.4°F (38°C) or higher. For children between 3 months and 3 years old, call their doctor if they have a temperature of 102.2°F (39°C) or higher.

Healthline