Virus

The enteroviruses are icosahedral, enveloped, single-stranded RNA viruses. Five types of enteroviruses cause diseases in humans, including the polioviruses (which cause polio) and echoviruses (which cause viral meningitis). Enteroviruses have an unusual replication cycle. They first enter the body through the upper respiratory tract and replicate within respiratory cells. Once the virus particles have been replicated, they are shed back into the oral secretions and are then swallowed. In the gastrointestinal tract, more replication takes place, and the viruses then enter the bloodstream. From the bloodstream, the viruses are carried to all parts of the body. However, further replication takes place only in cells for which the virus has an affinity. For example, polioviruses enter the nerve cells in the brain. There, viruses undergo further replication, and symptoms begin to appear.

Polio, the disease caused by polioviruses, is now controlled in the United States and other developed countries by the polio vaccine. However, before the introduction of this vaccine in the late 1950s, polio was one of the most feared human diseases. It causes a widespread paralysis of the muscles, sometimes including those of the respiratory system. Prolonged breathing assistance is needed for those individuals in which the respiratory muscles are paralyzed.

Echoviruses, another kind of enterovirus, cause viral meningitis, an inflammation of the nervous system. This disease is not as serious as the meningitis caused by bacteria, called bacterial meningitis.