How many days can measles be contagious before the rash appears?

Measles can be contagious from approximately four days before the rash appears until about four days after. This means that individuals infected with the measles virus can spread the disease for a total of about nine days, which encompasses a period starting from four days prior to the onset of the rash up to four days after the rash develops. This extensive contagious period underscores the importance of vaccination, as the measles virus is highly transmissible. Awareness of this timeframe is crucial for public health measures to prevent outbreaks and protect those who are unvaccinated or immunocompromised. Therefore, nine days is the correct duration when measles can be contagious before the rash manifests.