I've also seen the same studies that say we are being too careful now - and losing some of our resistance.
When we'd go to sea on a submarine, the hatches would slam shut and over 100 people would start inhaling (and smelling) everything that the other crewmembers experienced.
And if you think a toilet flush spews a few feces, just wait until one of your shipmates [-]stupidly[/-] [-]absent-mindedly[/-] accidentally opens a toilet flushing valve while the tank's contents are pressurized to over 100 PSI for blowing overboard. Even if it was just for a couple of seconds, they'd [-]catch a lot of sh!t[/-] not be able to contain the casualty. As almost everyone's noses could quickly tell, the submarine's highly efficient ventilation system would immediately whisk the particulate plume throughout the spaces within minutes.
Even without incidents like that, the week after the hatches went shut almost every one of the crew would suffer from some sort of rotavirus. (We knew it was an upper respiratory infection, but the corpsman didn't have the budget or the equipment to determine exactly what caused it.) Within the week, however, that would clear up.
After day 10 or so, nearly the entire crew would be healthy. From then on, with very rare exceptions there would be no colds, coughs, flu, or other respiratory infections. No alcohol or cigarettes, either (as far as we could tell) but plenty of chewing tobacco. This despite working 18-hour days with chronic fatigue and no sunshine (except for the periscope operators). During 90-day patrols you'd almost never see even a sniffle for well over two months.
But if we did even a personnel transfer, let alone bringing aboard more riders or pulling into port, then everyone would get another respiratory infection within the week. And if we went back out to sea then within another week everyone would be healthy again.
Admittedly we were among America's healthiest demographic, plus we were repeatedly inoculated for hepatitis & tetanus and tested for tuberculosis. Annual flu shots were mandatory, not optional.
But the immune system can only do its job if it has something to do its job to. Chronic exposure to contaminants and diseases appears to lead to a stronger immune system and, overall, a healthier body. If the body doesn't have a chance to learn how to cope with it, then it can be quickly overwhelmed and defeated. Look at what the original Americans and the Europeans did to each other with syphilis and measles.
So if your family & friends give you sh!t in more than merely the metaphorical sense, they're just doing it with your best immune-system health in mind.