An infant, seen in the Emergency room, presents with the fever and persistent cough. Physical examination and a chest X-ray suggest pneumonia. Which of the following is most likely the cause of this infection?

Respiratory syncytial virus (RSV) is the most impoant cause of pneumonia and bronchiolitis in infants. The infection is localized to the respiratory tract. The virus can be detected rapidly by immunofluorescence on smears of respiratory epithelium. In older children, the infection resembles the common cold. Aerosolized ribavirin is recommended for severely ill hospitalized infants. Rotavirus causes diarrhea. Coxsackievirus causes hand, foot, and, mouth disease.