Picornaviridae

The name combines pico, meaning small, with RNA. Being non-enveloped, they resist drying, detergents, alcohol, and bile, which supports faecal-oral spread and fomite transmission. The positive-sense genome is directly translatable, producing a single polyprotein that viral proteases cleave into structural and non-structural proteins. A helpful mnemonic for the members is PERCH: poliovirus, echovirus, rhinovirus, coxsackievirus, and hepatitis A virus. Two members break the family's usual pattern in ways examiners like: rhinovirus is acid-labile, so it is destroyed in the stomach and cannot spread faecal-orally, and it replicates best at the cooler temperature of the upper airway, which is why it causes the common cold rather than enteric disease; and hepatitis A causes acute hepatitis with no chronic carrier state, spread by contaminated food and water especially shellfish, and is prevented by an inactivated vaccine. Rhinovirus binds ICAM-1 and has over a hundred serotypes, which is why no vaccine exists. The enteroviruses share summer-autumn seasonality and cause aseptic meningitis.