Joe Schmo is your average college graduate who is suddenly transported back to medieval times, without time to prepare. From there the stories, and questions on this site, are all about how he can use his technological knowledge to change mankind. That assumes he survives long enough though...
Would Joe be able to survive the diseases and viruses of the time period? People from the US can't even drink water in Mexico because we are so dependent on our filtration, I can't imagine medieval water to be any cleaner. Would his immune system be able to handle the sudden influx of every-day diseases of the time which he has never been exposed to or developed an immunity to?
He would have vaccinations against some of the biggest killers, which is useful, but what about all the diseases he wasn't vaccinated against?
Now lets assume that Joe is smart, with a general high school and college bachelor's education. What can he do to protect himself? Would he have to start boiling all water before drinking it, until he can slowly build up a tolerance to their diseases?