Okay I am going to assume you guys have read the cough? For those who haven't:
Spoiler Alert::
In an apocalyptic world, where a horrible sickness is spread via cough and the only way to stop it is to kill the pony who coughed, The Mane 6 [i have NO idea where Spike is] lock themselves in a room with no windows and enough food and water to last them months.They are in pure darkness. It is the only safe place in Equestria. That is, until someone coughs.
The Mane 6 goes around asking each other who coughed. Fluttershy admits it's her and asks Rainbow Dash to deliver her a swift death. Rainbow ills her, and then they discover the truth, Fluttershy WASN'T the one who coughed
Spolier Ended
So, by reasoning, I can narrow this down to two ponies, I will leave the discussion up to you guys. I did process of elimination, so,yeah.
First off, Apple Jack is off the suspect list, being the element of honesty that she is, it would be impossible for her to say that she didn't do it.
Obviously Fluttershy is off as well, because, although she could of been the pony to cough first and then somepony else caught it, I don't think it spreads that quickly.
Third, Rainbow Dash. The meaning of loyalty is willingness to lay down your life for someone else's, so,no.
Then, Rarity. Genorosity means "showing a readiness to give more of something [i.e your life] than is expected." If Rarity DID cough, she would give herself up so her friends could live.
And that leaves two, Pinkie Pie and Twilight. These two are hard, because though both of them have one or two things that cold help, there is nothing that proves 100% that they are innocent.
First Twilight:
The Princess Of Friendship, well yeah but that doesn't mean she wouldn't lie to save herself.
Her element is magic, that does not help her with this, it couldn't be Rainbow Dash, Rarity,Fluttershy or Apple Jack, because their elements are the only evidence they need. If Apple Jack was accused she could say "I am the element of honesty, how could I lie?", but TWILIGHT can't say that.
Second Pinkie Pie
She wants to keep others happy, well, she may think that without her, nopony will be happy.
Also with Twilight, Pinkie Pie's element REALLY does not help her here.
So who do you think it is?