Croup is caused by a viral infection, most commonly from parainfluenza viruses (types 1 and 2), that spreads through respiratory droplets and contact with contaminated surfaces. A child catches it the same way they catch a cold: by breathing in droplets from a cough or sneeze, or by touching a surface where the virus has landed and then touching their nose, mouth, or eyes. The virus infects the lining of the upper airway, triggering swelling that produces croup’s signature barking cough.
The Viruses That Cause Croup
Parainfluenza viruses are responsible for most cases, but they aren’t the only culprits. Respiratory syncytial virus (RSV), influenza, adenovirus, and occasionally other respiratory viruses can all cause the same pattern of airway swelling. What these viruses have in common is that they target the cells lining the respiratory tract, replicating efficiently there without spreading to the rest of the body.
Once the virus takes hold, it triggers an immune response that causes inflammation and swelling in the larynx (voice box) and the area just below it. In young children, this part of the airway is already narrow, so even a small amount of swelling significantly reduces the space available for air to pass through. That narrowing is what creates the harsh, seal-like bark and the high-pitched sound (stridor) some children make when breathing in.
How the Virus Spreads
The transmission is straightforward. When an infected person coughs, sneezes, or even talks, tiny virus-laden droplets enter the air. Anyone nearby can inhale them. The virus also lands on toys, doorknobs, and other surfaces, where a child can pick it up on their hands. Daycare centers and preschools are common places for outbreaks because young children share space, share objects, and aren’t great about handwashing.
A child with croup is contagious for about three days after symptoms first appear, or until their fever is gone. The incubation period before symptoms show up is similar to a typical cold, usually two to six days after exposure.
Who Gets Croup and When
Croup overwhelmingly affects young children. It’s most common between six months and three years of age, affecting roughly 3% of children in that range. Cases can occur up to age six, and occasionally in younger infants, but the peak risk sits squarely in the toddler years. The reason is anatomy: a toddler’s airway is small and soft, so the same amount of viral swelling that barely bothers an older child or adult can dramatically narrow a toddler’s breathing passage. As children grow and their airways widen, croup becomes rare.
Croup can happen any time of year, but cases spike in fall and winter, tracking with the seasonal circulation of parainfluenza and other respiratory viruses. In the United States, croup accounts for about 7% of hospitalizations for fever or acute respiratory illness in children under five.
How Symptoms Develop
Croup doesn’t start with a barking cough. It typically begins like any other cold, with a runny or stuffy nose and possibly a low fever. Over the next 12 to 48 hours, the infection moves deeper into the airway, swelling intensifies, and the characteristic bark appears. Symptoms are almost always worse at night, which is why many parents first hear the cough after putting their child to bed.
Most cases are mild and resolve within three to five days, though the barking cough may linger a bit longer. The worst night is usually the first or second, and each subsequent night tends to be better than the last.
Why Cold Air Seems to Help
Parents often notice that stepping outside into cool night air eases their child’s breathing. This isn’t just anecdotal. Cold air activates receptors in the upper airway that increase the activity of muscles responsible for keeping the airway open, reducing resistance to airflow. Cold air also causes blood vessels in the airway lining to constrict, which temporarily shrinks the swollen tissue and opens up more space for air to pass through.
Humidity, on the other hand, doesn’t appear to help. A Cochrane review found that humidified air was no better than regular air at reducing croup symptoms after 30 to 60 minutes. Running a hot shower to create steam in the bathroom is a widely shared piece of advice, but the evidence doesn’t support it.
When Croup Keeps Coming Back
Some children get croup repeatedly, and this pattern deserves attention. One or two episodes are normal, especially for a toddler in daycare who catches every virus going around. But recurrent croup, where a child gets it several times a year or continues getting it past the typical age range, can signal something beyond a simple viral infection.
Gastroesophageal reflux (GERD) is one possibility. Stomach acid washing back into the esophagus can irritate and swell the same area of the airway, mimicking croup. Asthma and environmental or food allergies can also cause the airways to overreact, producing croup-like symptoms. In rarer cases, recurrent episodes point to a structural issue the child was born with, such as a narrowing below the vocal cords, floppy tissue in the voice box, or abnormal growths in the airway. Children’s Hospital of Philadelphia identifies these anatomical differences as important to investigate when croup recurs frequently.
Bacterial Tracheitis: A Rare but Serious Mimic
Occasionally, what looks like severe croup turns out to be bacterial tracheitis, a deeper infection of the windpipe. This condition often starts as a viral illness that seems to improve, then suddenly worsens with high fevers, toxic appearance, and significant breathing difficulty. It’s far less common than viral croup but more dangerous, typically requiring hospitalization. The key difference is the severity: a child with bacterial tracheitis looks much sicker than one with typical croup, and the condition doesn’t respond to the usual croup treatments.