Service connected

So I had a thought, and hoping sources with actual answers….not “this is my experience” or “well I think it should”

Even if a condition is started in the Military….but can’t be linked to an actual service event, does it still get counted as service connected?

The reason I pose this question has nothing to do with my claim (still in process) but as a general thought because I see a lot of “it started when I was in” cases and I have a hard time attributing that to a service connected…. So just a good for thought and I would like some insight for general poops and giggles.