the story: Harry is influenced by love to help out aliens trying to break away from a generational ship.
what it's all about: That whole "generational ship" thing is hugely relevant to Voyager but surprisingly not something the episode really seem to realize ("Prophecy" in the seventh season doesn't entirely miss the point, thankfully), but it's certainly worth noting as relevant to the series, and so it's entirely possible someone came up with the idea because of its implications for a Starfleet ship possibly headed to a similar future on its long way home.
Anyway, the real thrust of "The Disease" is an allegory for how love tends to screw around with your priorities, so that you don't necessarily think straight and start making questionable decisions. Tom Paris already had an experience earlier in the season ("Thirty Days") where he made a similar questionable decision, for different reasons, so there's a certain amount of resonance in that, too. Harry is the natural to star in this kind of episode, which again emphasizes the basic naiveté that's at the heart of the character, too young however experienced to understand that even something that seems right (again, in contrast to his pal Tom's adventures earlier) is probably still wrong (in Enterprise's "Cogenitor," Tucker does something similar for much the same reasons, but like "Thirty Days" it's an episode with more weight to it).
franchise- Pretty standard episodic material that will sway interest not at all.
- series - A possible glimpse at the ship's future, in some respects.
- character - Another chance to see Harry in context.
essential- Misses an opportunity to drive the generational concept home.