A virus is a small infectious organism—much smaller than a fungus or bacterium—that reproduces by invading a live cell. Virus infections are very common during childhood and adolescence. Clinicians will treat children and adolescents with infections caused by a variety of viral viruses on a frequent basis. These infections appear in a variety of ways. Many infections have clinical manifestations that are particular to the infecting virus but vary depending on the patient's age and immunocompetence. The majority of children who are infected with a viral recover without treatment. Fever and bodily pains or discomfort are common symptoms of viral infections. The majority of viral infections in children cause self-limited sickness that is managed symptomatically, and infected children recover completely.