I did receive a message asking to explain this but I typed it all up and tumblr had some error thing and it was all deleted.

So,

Basically the fact that Inara was dying was never explicitly stated in the show, comics or movie, but there were allusions:

• During the reaver encounter in the pilot Serenity, Inara takes out a syringe
• In Out of Gas, Simon says “I don’t want to die in here” and Inara replies, “I don’t want to die at all”
• In Heart of Gold, Nandi comments that Inara, “hasn’t aged a day” (possibly referring to the effects of whatever medication she is taking)
• In the comic Better Days, Simon is seen exiting Inara’s shuttle
• At the 2008 DragonCon panel, Morena Baccarin confirmed that Inara was dying of a terminal illness

I hope that clears things up a little!