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The most common cause of fever of unknown origin in children is viral infections. This is primarily due to the fact that viral infections are indeed prevalent among the pediatric population, often presenting with nonspecific symptoms such as fever. Many viral infections can exhibit a transient course where the child may present with fever but without clear localizing signs of infection, making it difficult to diagnose on first examination.

Additionally, in children, the immune system is still developing, making them more susceptible to various viral pathogens. Common viral illnesses that can cause prolonged fever include viral respiratory infections, infectious mononucleosis, and certain enteroviruses, among others.

While bacterial infections also cause fever, they tend to be more likely to present with localized findings that direct a healthcare provider to a specific diagnosis, making them less common as a cause of fever of unknown origin. Fungal and parasitic infections, while serious, are much less common in the pediatric population, especially in immunocompetent children, which further solidifies the predominance of viral etiologies in cases of unexplained fever.