A large part will be their sick policy. If kids come to school sick, they'll get everyone else sick. Plain and simple. Our preschool had an absolute "no sick children" policy, so he didn't get sick very often. The times he did were usually traceable to family contam... as in I or DH or nana gave him something... OR the kid at the store coughing up a lung on everyone, or smearing their runny nose all over the library books kind of thing ((It just drives me NUTS that parents bring their sick kids out in public instead of keeping them home... and I'm not talking the "I'm a single parent and have to run to the store" kind of thing... I'm talking the "I don't wanna be cooped up in a house with a sick kid, and don't give a rip about my child getting well faster or exposing other people" kind of thing)).
As soon as we hit K, he was sick at least twice a month, because our district has a 9 day per YEAR absence policy. There were kids in his class literally vomiting or running fevers over 102.5 who were not picked up by their parents until after "lunch" when the day counts as a non-absence. Ugh! ((Sorry, vent)).
So it all depends. While kiddo was in preschool DH and I were both at the U fulltime, and I was working in a hospital... so he was exposed to a lot, but it wasn't the constant-school-sick of so many preschools & elementary schools where the kids are shedding virus and bacteria by the gazillions ((A lot of HOW sick a kid will get will depend on the amount of exposure. If they only get a few hundred thousand germs from someone barely shedding, they might get a little sniffly... keep them home a day and they're fine... if they get a gazillion germs from being in an enclosed space from someone shedding like crazy because they are SICK sick sick... the massive exposure means that by the time the immune system develops an antibody they too are massively sick. This isn't true for ALL illnesses, but a vast majority of them, length and breadth of exposure directly correlates to whether they become mildly ill or severely ill))