Anytime you get groups of potential germ pots together there will be illness. Period. No cleaning protocols outsize of hazmat suits can be totally effective. So yes
DCL has its share of disease outbreaks.
My wife and I have been on five DCL cruises and have come home completely healthy each time (well perhaps not my liver). My wife and her sister went on one carnival cruise and both came down with norovirus. Conclusion? Nothing. It's anecdotal evidence. i.e. not reliable when applied to larger populations.
The question is are some conditions better than others so the likelihood of catching something is less? Yes. The size of the germ pool will contribute (bigger is worse), the length of exposure time (cruise) will contribute (longer is worse), the time of year (winter is worse), and perhaps the age stratification (kids tend to carry more germs than adults). And yes, finally, some cleaning protocols and enforcement are better that others. No doubt there are other parameters, but you get the idea, specifics matter
As has been mentioned, the CDC publishes useful data and on it DCL fares well. But, is this only because fewer ships means fewer listings over 3%? To answer that you need to normalize the data by number of cruises, size of the cruises, etc, and that's more than I care to do, but you can bet the cruise lines do it.
And so has US News and they wrote an article based on statistics about the 5 cleanest cruise lines in 2025
https://travel.usnews.com/features/top-cleanest-cruise-lines
And yes DCL makes the list! Interestingly enough, all five are considered premium cruise lines.
Perhaps quality does count.