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Acute bronchitis is primarily caused by viral infections, which account for the majority of cases, often following upper respiratory infections like the common cold. Bacterial infections can also contribute to acute bronchitis, though they are less common than viral causes. The inflammation of the bronchial tubes leads to symptoms such as coughing, wheezing, and difficulty breathing.

While fungi, allergens, and environmental pollutants can irritate the airways and potentially contribute to respiratory issues, they are not the main causes of acute bronchitis. Fungal infections are rare and typically associated with immunocompromised individuals rather than healthy people experiencing acute bronchitis. Allergens generally trigger allergic responses rather than infections, and environmental pollutants can exacerbate existing bronchial conditions but do not typically cause acute bronchitis in a healthy individual. Thus, the comprehensive understanding of the common etiologies of acute bronchitis aligns with the correct answer being bacteria or a virus.