Our biostats class started from the beginning, but people who'd had stats before were a lot more successful.
That said, I think it's debatable whether or not you need probability/statistics for med school, but you definitely need that knowledge to be a doctor.
For example, if your patient gets a positive result on a test, what does that do to your management? You have to know what the sensitivity and specificity of the test are AND what that means. Such as, how likely is it that your patient actually has the disease you tested for?
You also need familiarity with prob/stat concepts to evaluate research findings you'll be reading about and incorporate them into your practice. (And explain to your patients why the report they heard on CNN isn't exactly accurate.)