C

Measles patients remain contagious for roughly 4–5 days after the rash appears; therefore, public-health texts recommend keeping the patient separated from susceptibles for a full week to be certain the entire infectious period is covered. Studies have detected viable or RNA-positive virus up to 4–5 days after rash onset, supporting a 7-day isolation window [Bischoff 2016, PMID 26386428; Banerjee 2015, PMID 26110839].