What causes rubella?

Single viral strain. Rubella is caused by a virus with only 1 known strain & humans as the only known source. Infection is passed by contact with infected fluid on surfaces or in the air from a sneeze or cough. Immunization has reduced the case numbers when used, but is not 100% efective after 1 dose. Ease of international travel & transmission during incubation results in clusters of new cases every year.