Thus all three possible reading frames are read. In the case of parainfluenza 3, a fourth protein D is translated by insertion of two nontemplated G residues. As the concentration of N proteins builds up in the cells, these molecules are believed to associate either with the transcriptase or with nascent plus strands, preventing polyadenylation, termination and re-initiation, thus switching to the replication mode in which full length plus strands are made. These in turn serve as templates for RNA replication. Newly synthesized minus strands associate with N protein and transcriptase to produce nucleocapsids, which eventually associate with M protein at the apical surface of the membrane into which HN or F proteins have migrated, causing cells to fuse with adjacent uninfected cells.
Infection occurs via the respiratory tract and virus transported by macrophages to lymph nodes.
About six days after infection viremia occurs. Virions spread to all the epithelial surfaces of the body. About 9 days after exposure abrupt onset of illness with cough running nose, and inflamed conjunctivae. Immune response contributes to respiratory damage, malaise and fever. Occasionally central nervous system involvement.