Woh... I've never even thought of this and it's so simple! Just as you pointed out this is basically what happened when people from Europe started coming over to the 'new world' carrying all their germs they became immune to over in the big cities.

If we would go back in time we would probably make a lot of people sick from germs we are used to now that they would never have seen. As well as us being killed off from stuff that's not around anymore.

Great post mate! Looking forward to more :D