Viral Infections

Adenoviruses: Clinical Syndromes

Adenoviruses cause primary infection in children and, less commonly, adults. Reactivation of virus occurs in immunocompromised children and adults. Several distinct clinical syndromes are associated with adenovirus infection (Box 1). Acute pharyngitis is usually nonexudative but is associated with fever.

Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) produces a yearly epidemic in temperate climates. Most commonly, it causes bronchiolitis but can also cause upper respiratory infections, tracheobronchitis, and pneumonia (Table 1). In the United States, RSV activity is greatest from December through April.

Parainfluenza Virus

Parainfluenza is a ubiquitous virus. It is the primary cause of acute laryngotracheobronchitis (croup) in children aged 6 months to 3 years. It is capable of infecting the lower respiratory tract as well by manifesting as bronchiolitis or pneumonia. Outbreaks can follow regular epidemic patterns or be sporadic. Certain antigenic types (described below) do follow epidemic patterns.


Influenza is a highly contagious, acute, febrile respiratory illness caused by influenza A and B viruses. The hallmark of these viruses is their ability to undergo rapid ongoing antigenic change and to cause annual or near-annual epidemics of febrile respiratory disease affecting all age groups. In addition, the unpredictable emergence of new influenza A subtypes can lead to explosive global pandemics of disease.

Influenza Infection

There are no specific physical examination findings associated with influenza. The patient usually appears ill and has fever. A clear nasal discharge is common.


Rhinoviruses are the most frequent cause of the common cold. One hundred two serotypes have been identified by neutralization with specific antisera, and additional strains have been isolated but are not yet typed. Rhinoviruses can be transmitted by two mechanisms: aerosols and direct contact (eg, with contaminated hands or inanimate objects).


Enteroviruses are one of three types of picornaviruses that cause disease in humans. As the name indicates, picornaviruses are small (pico) ribonucleic acid (RNA) viruses that have a naked capsid structure. The family includes > 230 members divided into five genera but only three—enteroviruses, rhinoviruses, and hepatoviruses (hepatitis A virus)—cause human disease.

Enteroviruses: Clinical Syndromes

Polio vaccines and global eradication efforts have eliminated poliomyelitis from the Western Hemisphere and are expected to eliminate “wild” polio infections from the world in the near future. However, vaccine-associated cases of polio do occur. Paralytic poliomyelitis is characterized by an asymmetric flaccid paralysis with no sensory loss.