What Causes Fever?

Fever is a sign—not a disease in and of itself. It’s the body’s reaction to an assault, most often an infection, and it serves a good purpose. Why, then, should we fear a fever, especially a high fever? Because it could, in some cases, signal pending danger.

Except for rare cases (such as rheumatic diseases or certain cancers), fever indicates an infection. Infections are usually either viral or bacterial (they could also be fungal, parasitic, etc., but let’s stick to the basics here). In general, most viruses will run their course and your child’s immune system will eventually deal with and eliminate the germs that caused the fever.

In most cases, you’ll see other signs and symptoms that will help you localize where the infection is, and therefore what type of virus is causing the illness. Is there a runny nose and cough with the fever? This is usually an upper respiratory infection. Is there vomiting and diarrhea, with mild abdominal pain? This typically signals acute gastroenteritis, or the stomach flu.

If the fever is a natural response of the body, when should we worry? This often depends on the clinical symptoms and the age of the patient.

Fever in a Newborn

A newborn baby simply can’t fight off a bacterial infection, and therefore any fever above 100.4 degrees is an emergency if your baby is less than three months old. Don’t wait for the morning, and don’t just give Tylenol—call your pediatrician immediately. The risk of a bacterial infection is just too great. Bacteria that are normal for a woman to harbor (such as E. Coli and Group B Strep) can be deadly for a baby.

As mellow as you once were, having a newborn gives you license to be anal and neurotic. The hygiene hypothesis debates the value of exposure to germs, to build our defenses. That’s an interesting idea, but not for a newborn! You can go outside with your baby, but not to a 1-year-old’s birthday party or the mall. And everyone must wash their hands and refrain from kissing the baby—that includes your partner, your doctor, and that random stranger on the street who will try to touch your baby!

What if mommy gets sick? Breastfeed often and love your baby. It’s just not realistic to separate mom from baby. But if your best friend's 2 year old has a runny nose, it’s just not worth it to see them this week. After the third month, your baby’s immune system is much stronger, and it will specifically be able to localize an infection.

Please remember that your pediatrician is there for you during these times, and there is no such thing as a dumb or bad question. We have 24-hour on-call systems and daily walk-in visits exactly for these reasons—to help you care for your newborn.

Fever in an Older Child

Fever in an older child by itself is rarely an emergency. If your child complains of localized ear or throat pain, pain on urination, or right lower abdominal pain (don’t forget about that appendix), this could signal a bacterial infection. Although antibiotic use should be limited, there are times when you may need help to fight off an infection, and you should see your pediatrician within 24 hours. If there is any fast breathing, or if your child is lethargic and just not looking right, it’s time to see the doctor immediately. If there are any mental changes, or vomiting with neck stiffness, it’s a definite trip to the emergency room, to rule out meningitis.

As your child grows, his immune system matures, and there are more clinical cues to help us assess how sick the child is. The lack of clinical cues is another reason we are forced to investigate fevers in newborns with more rigor—babies can’t talk, play, or even smile when they are first born, so how do we know if they are very sick? An older child can look or act “out of it” with a high fever.

Regardless of how high the fever is, the approach is the same: First, bring the fever down with medicine, and then reassess (click here for dosages). If after administering Tylenol or Motrin your child is playful and happy and eating and drinking, this is a great sign. The infection is most likely viral, and you may not need to see your pediatrician. If, however, after the fever breaks your child still isn’t interactive or happy, it’s time to get him evaluated.

You Know Best!

Always remember that with any situation, you know your child best, and there should be a low threshold to making sure your child is okay. So if something just doesn’t feel right and you are concerned, go see your pediatrician. That is what we are here for!

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