Parainfluenza viruses cause about 75% of all croup cases, making them by far the most common trigger. Croup, known medically as laryngotracheobronchitis, is a viral infection of the upper airway that produces a distinctive barking cough, hoarse voice, and a high-pitched breathing sound called stridor. It primarily affects children between 6 months and 3 years old, though it can occur in kids as young as 3 months and as old as 15.
Parainfluenza: The Primary Cause
There are three types of human parainfluenza virus (HPIV) that matter here, and they behave differently. HPIV-1 is the most common cause of croup in children and tends to circulate in the fall. HPIV-2 also causes croup in the fall but is detected less frequently. HPIV-3 is more often linked to lower respiratory infections like bronchiolitis and pneumonia, though it can contribute to croup cases, especially in spring and early summer when the other two types aren’t circulating.
This seasonal pattern explains why croup cases tend to spike in autumn. When HPIV-1 and HPIV-2 are most active, emergency departments see a predictable wave of toddlers with that unmistakable seal-bark cough.
Other Viruses That Cause Croup
The remaining 25% of croup cases come from a range of other respiratory viruses. RSV (respiratory syncytial virus), influenza A and B, rhinoviruses, and adenoviruses can all produce croup symptoms. Measles is a rare cause but can lead to especially severe disease when it does.
COVID-19 has also entered the picture. During the Omicron wave, researchers studying hospitalized children with croup found that those who tested positive for COVID tended to be younger, had more severe symptoms, and stayed in the hospital longer. In that study, 73% of the COVID-positive croup patients had moderate to severe symptoms, compared to 32% in the COVID-negative group. These children also needed more rounds of treatment to get their symptoms under control.
Why Children’s Airways Are So Vulnerable
Croup viruses start by infecting the nose and throat, then spread downward to the voice box (larynx) and windpipe (trachea). The inflammation and swelling this causes wouldn’t be a big deal in an adult, but in a small child, it creates a serious bottleneck. The narrowest part of a child’s airway sits just below the vocal cords, where a ring of cartilage forms a rigid circle that can’t expand to compensate for swelling.
This is where physics works against young children. Resistance to airflow increases dramatically as the airway gets smaller. Even a small amount of swelling in a toddler’s already-tiny airway causes a large increase in the effort needed to breathe. Think of sucking on a straw that’s partially pinched: the harder you pull, the more the straw collapses inward. The same thing happens in a child’s soft, flexible airway walls during inspiration, which is why stridor is loudest when a child breathes in. As children grow and their airways widen, croup becomes less of a threat, which is why it’s uncommon after age six.
What Croup Looks and Sounds Like
Croup typically starts with a day or so of ordinary cold symptoms: runny nose, mild congestion, maybe some hoarseness. Then, often abruptly and usually at night, the barking cough appears. Children may develop stridor, a harsh, high-pitched sound with each breath in. Most children with croup have a low-grade fever, generally under 101°F, and don’t look seriously ill.
Symptoms tend to peak quickly. In two large observational studies, the vast majority of children were at their worst by the time they arrived at the emergency department. The good news is that croup resolves fast. Half of children see their symptoms clear within about two days, and 90% are better within three to five days. The barking cough is the last symptom to go, lingering for a median of roughly one and a half to two days, though it can persist for up to five days in some kids.
Croup vs. More Serious Infections
Most croup is mild and viral, but two bacterial conditions can look similar and are far more dangerous. Bacterial tracheitis causes similar airway symptoms but with high fevers (typically 102°F or higher) and a child who looks genuinely sick. It almost always develops on top of existing airway damage, such as during or after a viral infection. Epiglottitis, a bacterial infection of the flap that covers the windpipe, causes sudden high fever and severe breathing difficulty. Children with epiglottitis look pale, anxious, and tend to sit upright, leaning forward with their head tilted back in an instinctive attempt to keep their airway open.
The key distinction is how the child looks overall. A child with viral croup may sound alarming, especially at 2 a.m. when the barking cough echoes through the house, but they’re usually alert, interactive, and have only a mild fever. A child with bacterial tracheitis or epiglottitis looks toxic and is clearly in distress.
How Croup Is Diagnosed
Croup is diagnosed based on symptoms and a physical exam. The combination of a barking cough, hoarseness, and stridor in a young child during fall or early winter is distinctive enough that routine viral testing isn’t necessary. Doctors don’t typically need to identify which specific virus is responsible, because the treatment approach is the same regardless of whether parainfluenza, RSV, or another virus triggered it. Severity is assessed based on how hard the child is working to breathe, whether stridor is present only with activity or also at rest, and whether the chest wall is pulling inward with each breath.
Children with mild croup, where stridor occurs only when the child is upset or active, can generally be managed at home. Those with stridor at rest or visible chest retractions need evaluation and treatment. Scores of 5 or higher on clinical severity scales are associated with the need for hospital-level care.