Okay, quick summary to get things rolling:

-Ancestral (research) vessel arrives in system, experiments on star
(-Possible defensive maneuvers by coronal entities)
-Researchers trigger reaction in star, damage vessel, which crash-lands on nearby planet
-Leader (either original commander of ship/mission or other authority figure) vies for control of survivors post-crash; cultivates a following, religious doctrine, saintly status
-Schism of scientists v. religious? (The gist, at least)
-[i]X[/i]ty years pass... (Occasional flares which gain religious significance, rare encounters w/ marooned coronal entities?)
-Unknown increase in coronal entities' proximity to descendants causes epidemic; religious officials use concealed tech from crash to send out SOS, [i]Enterprise[/i] receives it

So are you saying you think the people who crash-land on the planet should be a mix of different species and/or perhaps part of an earlier Federation mission? That's an interesting idea - a bit like the [i]Bozeman[/i] in "Cause and Effect" (the one w/ Kelsey Grammer as a TOS movie-era captain) - but it could make things perhaps more complicated than they might be already. I'd definitely be up for revisiting the [i]Bozeman[/i] or that kind of scenario later, though!

The episode you're thinking of is "The Enemy." [i][url=http://www.imdb.com/title/tt0049223/]Forbidden Planet[/url][/i] is a '50s sci-fi movie starring Leslie Nielsen (among others). I definitely recommend it. ;)

You bring up some good points. A rough timeline would be good, as would some symptoms. I think the crash-landed party and their descendants need [i]at least[/i] 100 years to have developed into what the [i]Enterprise[/i] finds, maybe more. If the planet is especially hostile, lifespans and expectancy will probably be shorter and so more 'generations' may have passed than would on a civilized world.

Unfortunately, I can't think of much else for the illness other than radiation poisoning/sickness like in "Thine Own Self." Skin cancer, maybe, due to the profusion of UV rays that the marooned coronals exude wherever they go? Their presence (and/or their attempts to erect a "light bridge" planetside) might also ruin crops, deplete water sources and generally increase the surrounding temperature to intolerable degrees. Ooh, and perhaps some natural absorption process of local light waves does tricky things with eyesight, like blinds people or causes trippy, hallucinogenic light shows. Direct contact would probably sear or even disintegrate organic life. Anyway, I doubt it would be something contagious, but the natives might not know that.

Question: Should we address or handwave how large the population of original survivors would have to be to produce enough offspring to establish a sustainable society? I'm willing to handwave, but some at least perfunctory explanation would be nice.