I believe the sickness some people can get on board is probably very common on land too & you are just as likely to catch it at shopping centres or any other fairly busy venue-theatre etc.
I can tell you for a fact that things go through schools too, the kids fall like flies. Same sort of thing, I bet. You never, ever see it on the news but most schools go through at least one bad patch a year, usually in winter.
It's very visible when it happens on a ship as it is a stable population. That's why we notice it in schools too.
But you are right in that sometimes people who aren't 100% well go on board as they don't want to miss the cruise. Same thing happens in schools I can promise you. Kids routinely are sent in sick. They will tell you at 8.45am that they were vomiting that morning but mummy had to go to work and said they can just see the nurse at school. Which is not actually what a school nurse is for, she doesn't start till 10.30am in any case.
Even if parents do keep them home, they are often then taken along to shopping centres during the day to pick up groceries etc
So yes, if you are able to get away from people, into wide open empty spaces & countryside, you are less likely to catch stuff from other people.
The Australian outback would be ideal for your holiday, whip.