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Babies: How Can You Tell if Your Baby is Ill?

A change in behaviour is often a sign of illness in babies. If your baby is ill, he may cry more or have a change in activity level.

When to see a doctor

See your child's doctor right away if you notice any of the following signs:

  • a fever (for babies 3 months old or younger)
  • crying inconsolably
  • listless or limp
  • having convulsions (seizures)
  • fontanelle, the soft spot at the top of his head, is starting to swell
  • pain
  • purple splotches on his skin, or another type of rash
  • pale or flushed
  • problems breathing
  • refusing to breastfeed or drink from a bottle
  • appears to have trouble swallowing
  • vomiting or has diarrhea

In newborn babies and young infants three months of age or under, fever may be the first and only sign of a serious infection. If you notice a temperature that is even just slightly above the normal range, bring your newborn baby to the doctor as soon as possible. A normal temperature is 38°C (101°F) taken rectally or 37.5°C (99.5°F) taken under the armpit.

Behaviour changes

One of the first signs of illness in babies is a change in behaviour. The baby may cry more or have a change in activity level. Generally, if your baby is active when awake, feeding well, and can be comforted when crying, small differences in activity level or crying are normal. However, if your baby becomes lethargic or irritable, it may be time to see a doctor. Lethargy or irritability may be signs that an illness is present.


Lethargic or listless babies have little or no energy. They sleep longer than normal, and they may be difficult to wake for feedings. When awake, they are drowsy or sluggish; they are not alert and they do not pay attention to visual stimulation or sounds. Lethargy can develop slowly over time, and parents may have difficulty recognizing it.

Lethargy may be a sign of a common infection such as a cold, or a serious type of infection such as influenza or meningitis. Lethargy can be caused by heart conditions or blood diseases such as thalassemia. There are numerous other conditions that can cause lethargy, and lethargy is usually one of many symptoms associated with any particular condition. Therefore, if you notice that your baby is particularly lethargic or listless, bring him to the doctor for an examination. Treatment will depend on the particular condition that is making your baby lethargic and listless.


Crying is a baby’s only way of communicating. Over time, babies develop different cries depending on what they need: food, sleep, a diaper change, or a cuddle. Parents gradually learn to decipher their baby’s cries and respond accordingly. Usually parents can console their baby by providing what he needs and cuddling him. However, some babies may begin to cry inconsolably. This may be due to a condition called colic​, where babies may cry non-stop for three hours or so each evening. Colic begins shortly after birth and may continue for the first six weeks of life.

A baby who becomes very irritable, fretful, and fussy, with long crying periods, may be ill or in pain. The baby may also become quite jittery or start to tremble. Irritability may be a sign that your baby has constipation, abdominal pain, an earache, or a viral or bacterial infection. The cause of your baby’s irritability may simply be constipation but it might be something more serious. If your baby is irritable and continues to cry longer than normal, bring him to the doctor for an examination. Treatment will depend on the particular condition that is making your baby irritable.


It is important to realize that while fever is serious in newborn babies, it is not necessarily a bad thing if the baby is over three months of age. Fever is the body's way of fighting infection, so it is actually a good thing.

How to take your baby’s temperature

There are two ways to take your baby’s temperature: rectally or under his armpit. Do not use a mercury-filled thermometer. The most accurate method is the rectal way; however, many parents do not find this approach very appealing. Here are a few tips for taking your newborn baby’s temperature.

Measuring temperature rectally using an electronic thermometer:

  • It is much easier to take a baby’s temperature if two people are doing it.

  • Lay your baby on his back and bring his knees up over his abdomen.

  • Make sure the thermometer is clean.

  • Dip the thermometer in some water-soluble jelly.

  • Insert the thermometer into your newborn baby’s bottom, about 2.5 cm (1 inch) in.

  • Wait for the thermometer to take the reading. This is usually indicated by a beep. Read the temperature carefully and write it down in a notebook.

  • Clean the thermometer after use with soap and water.

  • The normal range for a temperature taken rectally is 36.6°C to 38°C (97.9°F to 101°F).

Measuring temperature under the armpit:

Armpit temperature
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An armpit temperature of 37.2°C (99°F) or higher means a fever.
  • Place the bulb of the thermometer in your baby’s armpit, and hold his arm down alongside his body. Make sure the bulb is completely covered in the armpit.

  • Wait for the thermometer to take the reading.

  • The normal range for a temperature taken under the armpit is 36.7 to 37.5°C (98.0 to 99.5°F).

Oral thermometers are not recommended until about age four. Ear thermometers should not be used in newborn babies and young infants because they tend to give inaccurate readings in the very young. Ear thermometers can be used over age two years. Fever strips, which are placed on the child’s forehead, are also not recommended because they are not accurate.

What causes fever?

Fever is usually a sign that your newborn baby’s body is fighting an infection. Bacteria and viruses usually thrive at a temperature near our normal body temperature. When we have a fever, our body temperature is elevated, which makes it harder for bacteria and viruses to survive. Fever also activates the immune system and sets the infection-fighting white blood cells into action. Usually, fever is associated with common illnesses such as colds, sore throat, or ear infections, but occasionally it can be a sign of something more serious.

Sometimes fever is not a response to illness, but rather it is caused by heat exhaustion or heat stroke. Heat exhaustion is a severe heat illness with symptoms of dehydration, fatigue, weakness, nausea, headache, and rapid breathing. It occurs when people in a hot climate do not drink enough water. Heat stroke is a life-threatening medical emergency where the body becomes so hot that is no longer able to regulate its temperature.

Treatment of fever in newborn babies

Do not try to use medication to treat a fever by yourself if your newborn baby is less than one month of age. Bring him to the doctor instead. The doctor might recommend that you give your newborn baby acetaminophen, but he will need to specify the exact dosage required.

In the meantime, you can continue to breastfeed or bottle feed your newborn baby as normal, if he will feed. If he is showing signs of dehydration, you may need to give him an electrolyte solution in between feedings, or replace the feedings with the electrolyte solution. Check with your doctor to make sure. The signs of dehydration include dry mouth, less than six wet diapers per day, tearless and sunken eyes, a sunken fontanelle, and dry skin.

You can also try giving your newborn baby a sponge bath with lukewarm water. If you let the water evaporate off his skin, it will help to cool him down. Do not add alcohol to the water.

When a newborn baby develops a fever associated with infection, it can become a great cause for concern. This is because newborn babies can get sick very fast. Luckily, they also respond very quickly to treatment, if the infection is caught in time. This is why it is important to bring your newborn baby to the doctor as soon as possible. If a doctor suspects that a newborn baby has an infection, he may begin antibiotic treatment right away.

Treatment of fever in older babies

Most fevers are caused by viruses and will get better without treatment. Because of this, many doctors do not recommend reducing a fever in infants over six months of age unless the fever is over 38.5°C (101.5°F). However, if the infant is having aches and pains from the fever, acetaminophen can be used to make him feel more comfortable.

If a fever is found to be caused by a bacterial infection, the infection should be treated with antibiotics. Antibiotics work to destroy the bacteria, and in the process lower the fever. Sometimes antibiotics and acetaminophen​ are used simultaneously to treat the fever. Fevers that shoot up past 41.5°C (106.7°F) are rare and should be treated immediately.

If a fever is due to heat exhaustion or heat stroke, it can be dangerous and requires immediate attention. Heat exhaustion can be treated by bringing the child indoors, loosening his clothing, encouraging him to eat and drink, and giving him a cool bath. Heat stroke is a medical emergency and should be treated immediately by a physician. While waiting for medical help, bring your child indoors, remove his clothing, and sponge him with cool water.

Douglas Campbell, MD, FRCPC


Hazel Pleasants, RN, MN

Andrew James, MBChB, FRACP, FRCPC