A relational database might be a better choice, but you could get some mileage from Devonthink. You want the correlations to just appear. You don’t want to manually maintain any more than you have to.
Here’s what comes to mind. Turn off “exclude groups from tagging” in File->Database properties.
Make groups for Diseases, Symptoms, Lab findings, and Examination findings.
When you add something, consider putting it in a specific group under one of those main topics, even if you add a new group for just one file.
So, let’s say you have a group called IQ of a Bivalve under symptoms. In there you’ve got notes about IQ testing, desire to eat crayons, belief we never went to the Moon, and whatever else goes with mollusk-level cognition.
Under Diseases, you’ve got a group for the terrible outbreak of Dunning-Kruger Upper Left.
If you tag IQ of a Bivalve with Dunning-Kruger Upper Left, the D-K group will sprout a subgroup, IQ of a Bivalve, which is a second instance of your Bivalve IQ symptom group.
As you add or edit notes about bivalves to either appearance of the IQ of a Bivalve group, the changes appear in both instances.
The D-K group shows everything with a connection to D-K. If you look at the symptom group, IQ of a Bivalve, the list of tags shows what it relates to. There, you can use the reveal tag feature to navigate to D-K Upper Left, Capitol Storming Syndrome, Higher Education Via Facebook, or any of the other diseases you’ve tagged as presenting with sub-vertebrate brain wattage.
Hope that helps. Please excuse any slant that may have come through in my comments. I just read a few posts elsewhere from some ardent science-deniers. Ewww.
No offense intended to octopuses. They are smart, and they are, in truth, mollusks.