What would you do in the year 1000AD?

[Smallville]

There was plenty of moss around back then... Wouldn't take much to make a batch of penicillin

Foodstuff, actually, to produce the blue-green mold. And then you'd need microscopic confirmation of the mold to be certain it was penicillin.  There are a lot of different molds that grow on foods. Then you'd need to create a way of injecting it for the productive effect.