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Croup is primarily caused by viral infections, with the most common pathogens being the parainfluenza virus, although other viruses such as adenovirus, respiratory syncytial virus (RSV), and rhinovirus can also play a role. The illness is characterized by inflammation of the larynx, trachea, and bronchi, leading to the hallmark symptoms of a "barking" cough, hoarseness, and stridor due to the narrowing of the upper airway.

The other types of infections—bacterial, fungal, and parasitic—typically do not cause croup. While bacterial infections can lead to conditions like epiglottitis, which can mimic croup-like symptoms, they are not the primary pathogens responsible for the classic presentation of croup. Fungal infections are rare in respiratory illnesses and are usually associated with immunocompromised patients. Parasitic infections typically involve gastrointestinal or systemic issues rather than respiratory illnesses like croup. Thus, the understanding that croup is primarily viral is essential for correctly diagnosing and managing the condition.