A baby gets croup by catching a common respiratory virus, most often through infected droplets in the air or by touching a contaminated surface. The virus then travels from the nose and throat down to the voice box and windpipe, where it causes swelling that partially blocks the airway. Because a baby’s airway is so small to begin with, even minor swelling produces the distinctive “barking seal” cough that sends many parents to the emergency room.
The Viruses That Cause Croup
Croup isn’t one specific illness. It’s the result of a viral infection that happens to settle in a particular part of the airway. Viruses are detected in about 80% of croup cases with identifiable causes, and parainfluenza virus (types 1 through 3) is responsible for roughly 75% of all cases. Human parainfluenza virus type 1 is the single most common culprit. Other viruses that can trigger croup include influenza A and B, respiratory syncytial virus (RSV), adenovirus, rhinovirus, and enterovirus.
These are the same viruses that cause ordinary colds and upper respiratory infections in older children and adults. The difference is what happens when they infect a baby’s much smaller airway.
How the Virus Spreads
Your baby can catch the virus in two main ways. The first is breathing in respiratory droplets that an infected person has coughed or sneezed into the air. Talking, singing, and even normal breathing can release these droplets, which makes close contact with a sick child or adult the most common route of transmission.
The second route is surface contact. Virus particles from respiratory droplets can survive on toys, doorknobs, highchair trays, and other surfaces. If your baby touches a contaminated surface and then puts their fingers in their mouth, nose, or eyes, the virus can take hold. This is a big reason croup spreads easily in daycare settings, where young children share toys and are in close quarters with other kids who may be sneezing, coughing, or drooling.
What Happens Inside the Airway
Once the virus enters the body, it starts in the nose and throat, just like a regular cold. Over the next few days, it moves deeper, settling into the larynx (voice box) and trachea (windpipe). The lining of these structures becomes inflamed and swollen, and the airway narrows.
Here’s why this matters so much in babies: a full-term newborn’s airway just below the vocal cords has an average diameter of only 4.5 to 5.5 millimeters. That’s roughly the width of a pencil. In premature infants, it’s even smaller, around 3.5 millimeters. When the airway lining swells by just one millimeter on each side, a baby loses a significant percentage of that already tiny opening. The same amount of swelling in an older child or adult, whose airway is much wider, barely causes symptoms. In a baby, it can produce noisy breathing, a harsh barking cough, and a high-pitched sound when inhaling called stridor.
This is the core reason croup is overwhelmingly a disease of young children. It’s not that babies catch different viruses. Their airways are simply too narrow to tolerate the same degree of swelling that an adult shrugs off as a sore throat.
The Timeline From Exposure to Barking Cough
The incubation period for croup is typically 2 to 6 days after exposure to the virus. During the first day or two of symptoms, croup usually looks like an ordinary cold: runny nose, mild fever, maybe some fussiness. The characteristic barking cough and noisy breathing tend to develop as the infection moves deeper into the airway, often appearing on the second or third day of illness.
Symptoms are almost always worse at night. Many parents describe a child who seemed to have a mild cold during the day and then woke up in the middle of the night with a sudden, alarming cough that sounds like a seal barking. Cool night air and the body’s natural drop in cortisol levels overnight both contribute to this pattern. Most cases of croup last three to five days before the swelling gradually subsides.
Why Some Babies Are More Likely to Get It
Croup is most common in children under 3 years old, with the peak hitting between 6 months and 2 years. Several factors raise the chances your baby will encounter the viruses that cause it:
- Daycare or group childcare: Close contact with other young children who are coughing, sneezing, and sharing toys creates constant exposure to respiratory viruses.
- Older siblings: School-age children bring home viruses frequently. They may have mild cold symptoms while the same virus produces croup in a younger sibling with a smaller airway.
- Fall and early winter timing: Croup cases peak in late fall in the Northern Hemisphere, tracking closely with the parainfluenza virus season.
Some children seem to get croup repeatedly, while others never develop it despite catching the same viruses. Part of this comes down to individual airway anatomy. A child born with a slightly narrower airway is more vulnerable to the effects of even mild swelling.
Spasmodic Croup: A Different Trigger
Not every case of croup starts with a virus. Spasmodic croup looks and sounds very similar to the infectious kind, with the same barking cough and stridor, but it isn’t directly caused by infection. It tends to run in families and may be triggered by an allergic reaction. A child with spasmodic croup often has no fever, no runny nose, and no other signs of a cold. The croupy cough can appear suddenly, typically at night, and may resolve just as quickly.
An infection can sometimes set off a spasmodic episode, but the underlying mechanism is different. If your child keeps getting croup-like symptoms without clearly being sick, this may be what’s happening.
How Long a Child Stays Contagious
Your baby is contagious during the first few days of symptoms, when the virus is actively shedding. The contagious window generally overlaps with the period when cold-like symptoms are present, particularly the runny nose and cough. By the time the barking cough is fading, your child is usually much less likely to spread the virus to others.
Keep in mind that the virus your child passes along won’t necessarily cause croup in the next person. An older child or adult who catches the same parainfluenza virus will typically just develop a cold. It’s only when that virus meets a young child’s narrow airway that croup develops.