The intuitive answer is 99 percent, but the correct answer is 50 percent, and Bayes's theorem gives us the relationship between what we know and what we want to know in this problem.

Bayes’ Theorem As explained in the FDA’s Guidance document, prior information about a topic that you wish to investigate in more detail can be combined with new data using Bayes’ Theorem ...

The theorem was the work of Thomas Bayes, a minister and statistician, whose formula was published posthumously in 1763 in the paper "An Essay Towards Solving a Problem in the Doctrine of Chances." ...