Picornavirus

The name combines pico, meaning small, with RNA — an accurate summary of the family. A picornavirus is roughly 30 nm across, has an icosahedral capsid, and carries positive-sense single-stranded RNA. Because the genome reads like messenger RNA, it can be translated by host ribosomes the moment it enters the cytoplasm, and purified genomic RNA is infectious on its own. Translation produces one long polyprotein that viral proteases cleave into the structural and nonstructural proteins.

Five members are worth memorizing, often with the mnemonic PERCH: Poliovirus, Echovirus, Rhinovirus, Coxsackievirus, and Hepatitis A virus. All except rhinovirus and hepatitis A virus are classified as enteroviruses and can cause aseptic meningitis. Poliovirus targets anterior horn motor neurons and causes flaccid paralysis; coxsackievirus A causes hand-foot-and-mouth disease and herpangina, while coxsackievirus B causes myocarditis, pericarditis, and pleurodynia; hepatitis A causes acute hepatitis with no chronic carrier state.

Lacking an envelope makes these viruses hardy. They resist detergents, drying, and — for the enteroviruses and hepatitis A — the acidity of the stomach, which allows fecal-oral transmission and survival in contaminated water and food. Rhinovirus is the exception: it is acid-labile, so it cannot survive gastric passage, and it replicates best at about 33 °C, matching the cooler upper airway. That combination explains why rhinovirus is the leading cause of the common cold and spreads by respiratory droplets and fomites rather than the fecal-oral route.

Picornaviruses are a reliable USMLE Step 1 virology topic. Know the genome type and capsid structure, the PERCH members, which ones cause aseptic meningitis, and the two features that set rhinovirus apart — acid lability and its preference for cooler temperatures.