Rubella is a contagious viral infection characterized by a rash that mainly affects the skin and lymph nodes. It is also commonly known as German measles. The "R" in the M.M.R. vaccine refers to rubella, indicating that the vaccine provides protection against this particular virus.
Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: The "R" of the M.M.R. vaccine. Explanations are generated by AI and may be inaccurate.