Paramyxoviridae

Paramyxoviridae is a family of enveloped viruses with negative-sense, single-stranded RNA genomes. A defining structural point — and a favorite exam contrast — is that the paramyxovirus genome is nonsegmented, one continuous strand, whereas the orthomyxoviruses (influenza) carry segmented genomes that allow reassortment. Paramyxoviruses replicate in the cytoplasm and spread primarily through respiratory droplets.

The family's classic human pathogens are measles (rubeola) virus, mumps virus, and the parainfluenza viruses; respiratory syncytial virus (RSV) and human metapneumovirus were traditionally grouped here as well before being reclassified into the related Pneumoviridae family. Many members carry a fusion (F) protein that fuses infected cells into multinucleated giant cells called syncytia.

Each virus has high-yield associations. Parainfluenza causes croup (laryngotracheobronchitis) with its seal-bark cough and steeple sign on neck X-ray. Measles presents with cough, coryza, conjunctivitis, and Koplik spots before its descending rash, and can cause the late complication subacute sclerosing panencephalitis. Mumps causes parotitis and can involve the testes (orchitis) and meninges. Because measles and mumps each have a single serotype, the live attenuated MMR vaccine provides durable protection.

The USMLE Step 1 tests Paramyxoviridae in its virology material — expect questions linking genome structure (negative-sense, nonsegmented RNA), the F protein and syncytia formation, and the clinical syndromes of measles, mumps, parainfluenza, and RSV.