Croup starts with what looks like an ordinary cold: a runny nose, mild sore throat, and a low-grade fever. Within one to two days, the infection moves deeper into the airway, and the signature barking cough appears, often suddenly and frequently in the middle of the night. That shift from “just a cold” to a harsh, seal-like cough is what catches most parents off guard.
The Virus That Triggers It
Croup is most commonly caused by parainfluenza viruses, though respiratory syncytial virus (RSV), influenza, and adenovirus can also be responsible. Your child picks up the virus the same way they catch any cold: through airborne droplets or by touching a contaminated surface and then touching their nose or mouth. The incubation period is two to seven days, meaning symptoms won’t appear right away after exposure.
Cases occur year-round, but hospital admission rates peak in late autumn, roughly September through December.
What Happens Inside the Airway
Once the virus takes hold, it causes swelling and irritation in three key areas: the voice box (larynx), the windpipe (trachea), and the bronchial tubes leading to the lungs. The voice box area is the bottleneck. When a cough forces air through that narrowed passage, the swollen vocal cords vibrate in a way that produces the unmistakable barking sound.
The high-pitched whistling noise some children make when breathing in, called stridor, happens through the same mechanism. Air moving through a tight, flexible airway drops in lateral pressure (a principle from physics called the Bernoulli effect), which can cause the walls of the airway to briefly collapse inward. That turbulent airflow creates the harsh whistling. In mild cases, stridor only shows up when a child is crying or coughing. In moderate to severe cases, it’s audible even at rest.
Why Young Children Are Hit Hardest
Croup is most common in children between 3 months and 5 years old. The reason is straightforward: their airways are physically smaller. A small amount of swelling that an older child or adult would barely notice can dramatically narrow a toddler’s airway. An adult with the same virus typically gets nothing more than a hoarse voice or a sore throat.
The Typical Timeline
Here’s what the progression usually looks like:
- Days 1 to 2: Runny nose, mild cough, possible sore throat. Fever is generally low-grade (around 100 to 102°F) but can occasionally climb higher.
- Days 2 to 3: Hoarseness develops. The barking cough and stridor appear, often abruptly, sometimes waking a child from sleep.
- Days 3 to 5: Symptoms typically peak around day two or three of the barking cough, then gradually improve.
- Days 5 to 7: The barking quality fades, though a residual cough can linger for a few more days.
Most cases resolve on their own within a week. The illness tends to be worst on the second and third nights.
Why It Gets Worse at Night
Parents commonly describe the same pattern: the child seems mostly fine during the day, then the cough turns harsh and frightening after bedtime. Several factors contribute. Lying down changes how mucus drains and how gravity acts on an already swollen airway. Cortisol, the body’s natural anti-inflammatory hormone, dips to its lowest levels overnight, which may allow swelling to worsen. The result is that croup can sound alarming at 2 a.m. and seem much milder by morning.
Cool night air can actually help. Animal research has shown that cooling the upper airway increases the activity of muscles that hold the airway open and reduces blood flow to swollen tissue, both of which ease the obstruction. Opening a window or stepping outside briefly on a cool night is a well-established home strategy for this reason.
Mild, Moderate, and Severe Croup
Not every case of croup looks the same. Clinicians grade severity based on a few observable signs, and you can use the same cues at home to gauge how your child is doing.
- Mild: An occasional barking cough with no stridor when the child is calm. Breathing looks comfortable. This describes the majority of croup cases.
- Moderate: Frequent barking cough with stridor audible at rest. You may notice mild retractions, where the skin pulls inward between or below the ribs with each breath. The child is not overly distressed.
- Severe: Frequent barking cough, stridor at rest, and visible retractions that are pronounced. The child appears agitated and is working hard to breathe.
Signs of a true emergency include a blue or gray tint around the lips or fingertips, extreme difficulty breathing, drooling or an inability to swallow, and a child who becomes unusually drowsy or unresponsive. These suggest the airway obstruction is critical and the child needs immediate medical care.
What Helps at Home
For mild croup, comfort measures make a real difference. Keep your child calm, because crying and agitation increase the demand for air through an already tight passage, which worsens stridor. Sitting upright or propping them up with pillows can ease breathing. Cool night air, whether from a cracked window or a brief walk outside, often provides quick, noticeable relief.
Humidity from a steamy bathroom is a generations-old remedy. Evidence for its effectiveness is limited, but many parents find it soothing in the moment. Keeping your child well-hydrated with small, frequent sips also helps thin mucus and reduce irritation. Over-the-counter fever reducers can manage discomfort from fever.
If your child has stridor at rest, visible retractions, or seems to be getting worse rather than better by the second or third night, that warrants a medical evaluation. Treatment for moderate to severe cases focuses on reducing airway swelling quickly, and the improvement is usually dramatic once that swelling comes down.