Measles is a highly contagious viral infection caused by the measles virus (MeV), belonging to the Paramyxovirus family. It primarily affects the respiratory system and is characterized by a distinctive rash. Measles was once a common childhood illness worldwide, but widespread vaccination efforts have significantly reduced its prevalence in many parts of the world.

The virus is transmitted through respiratory droplets when an infected person coughs or sneezes. It can also be spread by direct contact with nasal or throat secretions of infected individuals. Measles is most contagious during the early stages of the illness, even before the rash appears, and remains contagious for several days after the rash develops.