1^N is "1 raised to the N". The one measles case so far growing exponentially where N is the exponent. It's still 1 for all values of N though. Nothing is actually increasing even though it's "growing exponentially". There is one measles case so far. Even if it "spreads exponentially" you would still only ever have one case if you intentionally misunderstand how disease spreads and model the number of cases with the function X^N where X is your starting number of cases.

Jokes aren't funny when you have to explain them though