Well, I know about hygiene about various diseases, and even how to treat them. I’m also running for my life to the nearest forest, as I don’t have any way to prove I’m not a Vagabound or invader, and I can’t communicate.

There’s no choice but to try to hide deep inside a forest, and try to find a river or lake. I would eventually be forced to drink dirty, risking parasites, unless I get lucky enough to find what I need to purify it.

As soon as that is taken care of, I would focus on covering up my tracks, digging some sort of shelter to hide from medieval people, my next step would be to ensure a food source like fishing, or finding plants I recognize as edible, which might be where I mess up and die (A lot of plants look similar to poisonous ones, mushrooms too).

Next step would be making bog iron, finding copper, and so on, to make life easier at first, but then to defend myself against being discovered.