Is it a Bacterial Infection or Virus?

Updated November 21, 2019

Bacterial infections are caused by bacteria, while viral infections are caused by viruses. That's the easy part. Differentiating between the two requires medical intervention since both may cause fever and irritability. And the treatments vary significantly. Pediatrician Betty Staples, MD, offers advice on how to tell the difference between these two types of infection.

Every day, parents bring their children to the pediatrician for help in determining whether their sick child has "just a cold" or something more.

Children's colds result in 22 million missed school days and 20 million parental missed days of work every year. In most cases, these are the "just a cold" variety of virus. However, we also know that other, less common infections can develop in our children, and these need evaluation by the pediatrician to determine if antibiotics are required.

Viral Infections

Common viral infections such as an upper respiratory infection can typically be detected by a runny nose, cough, low-grade fever, sore throat, and difficulty sleeping. No antibiotics or anti-viral medications can hasten recovery from the cold.

Of note, when compared to adults, upper respiratory infections in children may last longer (up to 14 days) and occur more frequently (average six to eight per year).

Influenza is a viral illness that can cause many of the same symptoms but also is frequently accompanied by intense body aches and higher fever. Unlike upper respiratory infections, the flu's duration -- if detected within the first 48 hours of illness -- can be shortened by antiviral medication.

A dose of flu vaccine (or two doses a month apart in the young child receiving flu vaccine for the first time) given at the start of each "flu season" can help to prevent influenza infections.

Bacterial Infections

In some cases we become more concerned that the infection may be caused by a bacterial infection. Bacterial infections may be the result of "secondary infection" (meaning that the virus initiated the process but a bacteria followed) when the:

  • Symptoms persist longer than the expected 10-14 days a virus tends to last
  • Fever is higher than one might typically expect from a virus
  • Fever gets worse a few days into the illness rather than improving

Sinusitis, ear infections, and pneumonias are common examples of secondary infections. For example, a runny nose that persists beyond 10-14 days may be a sinus infection that would be best treated with an antibiotic. Ear pain and new onset fever after several days of a runny nose is probably an ear infection. Depending on your child's age, these infections may or may not require an antibiotic.

Pneumonia may be detected by persistent cough, stomach ache, or difficulty breathing. Your physician may diagnose pneumonia by physical exam or may request a chest x-ray.

Other bacterial illnesses that we are concerned about include urinary tract infections (UTIs), which can be hard to detect and can cause kidney damage if they are untreated. If your child has a fever without a great source of infection, your doctor will likely want to check the urine. UTIs are more common in little girls and in baby boys under one year of age who are not circumcised.

More serious concerns are bacterial illnesses like sepsis (bacteria in the blood) and bacterial meningitis (bacterial infection in the lining of the brain and spinal cord). We become concerned about meningitis in older children with a stiff neck or changes in mental status. Babies are less likely to be able to show us these symptoms, and we are more likely to do more tests on them to make sure these infections are not part of the illness.

Remember that many of the vaccines that your child receives in the first years are meant to prevent these serious bacterial infections.

Diagnosing Bacterial Infection

Tests that are frequently performed to help us with the diagnosis of a bacterial infection include a complete blood count and cultures of fluid that we are concerned about. This may include a blood culture, urine culture, or spinal culture (which requires a spinal tap).

Whether the infection turns out to be caused by virus or bacteria, you should watch your child for any of the following signs and bring them to medical attention if they develop:

  • Dehydration, demonstrated by decreased fluid intake; urination less than three times in 24 hours; or decreased tears with crying
  • Increased work of breathing including fast breathing, nostril flaring, use of rib, stomach, or neck muscles to breathe
  • Markedly decreased activity or responsiveness
  • No improvement over a three - to five-day period
  • All children under three months of age with a fever

Children who are around other children will have more frequent infections. But remember most children these days (thanks to vaccines that prevent most serious secondary bacterial infections) will have viruses that take supportive care only.

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