capacity to fuse membranes and is responsible both for the formation of syncytia and for entry of the virus into the cell (Scheid and Choppin, 1974). Antibody to either glycoprotein is neutralizing, and antibody to the fusion protein also prevents cell-to-cell spread of the virus (Merz et al., 1980). The fusion proteins of measles and mumps viruses, closely related to paramyxoviruses, are antigenically denatured by formalin, and it is possible that early measles and parainfluenza vaccines failed for this reason.
Reinfections are common with PIV-1, PIV-2, and PIV-3. They appear to be most frequent with PIV-3 (Chanock et al., 1963). In addition, evidence from volunteer studies in adults suggests that secretory neutralizing antibody correlates better than serum antibody with protection against challenge with PIV-1 (Smith et al., 1966). It is assumed that this rule also holds for PIV-2 and PIV-3, although the assumption has not been proved, and information in children is scanty.
Although there is some cross-reactivity between these three PIV types, cross-protection probably does not occur. There does not appear to be any serologic variation within each type.
Seroepidemiological studies indicate that the parainfluenza viruses are ubiquitous; even isolated populations have been found to possess antibodies against them. The severity of diseases caused by the viruses and the age of initial infection may vary in different parts of the world, but little information is available on these topics.
Croup appears to be less common in the developing world than in developed countries, but the reasons for this situation are not known: it is possible that the PIV infection simply takes a different clinical form. Also, it is unclear to what extent PIV infections play a role in the life-threatening respiratory diseases often seen in children in developing countries.
Examining mortality statistics provides some perspective on the burden of parainfluenza virus infection in children in developing countries, especially those under age 5. The causes of childhood mortality often change as a country develops. Initially, diarrheal diseases may be the leading cause of death. General development and the implementation of oral rehydration programs may reduce the impact of these diseases and increase the proportion of deaths due to respiratory infections. With further development, mortality from acute