I definitely understand your question and concern.... not trying to be funny but the simple answer to your question is that for as long as cruising has been around... I am pretty sure that at least one person got back home sick.... but then again, a lot of people don't.
Case in point... we sailed 4 times in 2022 and even though there were people around us, especially in August, that weren't feeling well... we were just fine... Yet, when we sailed in 2018, 2019 and 2020 someone in our party would always come back home with a little something (and for all we know, 2020 could have been COVID).
All that to say, there are pretty good responses above around measures one can take while on the ship, in the cruise terminal, in planes and airports and so on, to minimize the odds of getting something.
I know people over the last few weeks who sailed who got sick... and some who didn't