Actually, it is just the opposite. By sharing those bodily fluids, their bodies build up resistance. Your body needs a certain level of exposure to "bad things" so it can handle other bad things. When I was deployed to third world nations, the local people had a resistance to pest and pestilence that we could only aspire towards.
When I served on the drill field, it was common for new drill instructors to be constantly sick for the first 2-3 months, because of all of the germs that the recruits brought from all over America. After that 2-3 month period, these new drill instructors very rarely got sick. When my kids were young, hand cleaner was all the rage. I did not let my kids buy or bring it home. The immunity that you get when you are young, helps you later in life.
I grew up with three siblings. All of us seem to have a high resistance to disease- we typically don't get sick and when we do, we have mild symptoms. Two of us married spouses that do get sick regularly. The two of us that married spouses (my wife and my sister's husband) that get sick regularly had two children each. Both my sister and I have one child that does not get sick and another child that gets sick. Since we raised our kids about the same (hose them down before dinner), I have no other explanation for the difference.
I was kinda joking about college students’ hygiene habits. Of course it’s true that building a healthy immune system requires exposure to bad things. Kids playing in dirt, that sort of thing. Hand sanitizer is still new to our state of evolution.
Even saw a recipe for `steak americaine’ (ground beef served raw) where the translation from French said, `for people with healthy immune systems or if you want to strengthen your own’.
On that topic, google “cannibal sandwich” (a Wisconsin favorite). The views run from prudence to prohibition.