People get rubella by breathing in saliva or mucus droplets from an infected person when s/he coughs, sneezes, or talks. An individual with rubella is most infectious when developing rash and infected persons can shed the virus from 7 days before to 7 days after onset of the rash.
https://www.nicd.ac.za/increase-in-rubella-cases-in-the-western-cape-province-17-october-2023/