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This transmission electron micrograph (TEM) revealed some of the nucleocapsid morphologic features displayed by the human parainfluenza virus Type-4a (HPIV-4), a member of the Paramyxoviridae family. These viruses possess a genome consisting of negative-sense single-stranded RNA ((-) ssRNA).

Each of the four HPIVs has different clinical and epidemiologic features. The most distinctive clinical feature of HPIV-1 and HPIV-2 is croup (i.e., laryngotracheobronchitis); HPIV-1 is the leading cause of croup in children, whereas HPIV-2 is less frequently detected. Both HPIV-1 and -2 can cause other upper and lower respiratory tract illnesses. HPIV-3 is more often associated with bronchiolitis and pneumonia. HPIV-4 is infrequently detected, possibly because it is less likely to cause severe disease. The incubation period for HPIVs is generally from 1 to 7 days.