Adult immune systems ‘remember’ germs to which they’ve never been exposed (2013)
I grew up on a farm.  It was common knowledge that when a calf, or a lamb was born they needed to get that first milk from their mom.  The first milk looked different and was called colostrum.  Colostrum contained a large assortment of antibodies from the mother that were transmitted to the newborn.  If there was some health issue, the mother died or the newborn couldn't drink, we had colostrum in the freezer.  If we didn't give it, the odds dropped dramatically of the newborn making it more than a few months.So, yes, mammals, including people, get immunities to germs they have not been exposed to.