Applejack is a sturdy pony. Why, her immune system is so strong, it's been said that diseases run in terror from her. Apparently, magical blue cerulean diseases follow a different set of rules.
Rarity, on the other hand, has hardly ever been sick before, mainly because she avoids disease like… well, a disease. Let's just say she doesn't take illness well.
It might take a little more than cold medicine to get better in this case.

Tagged 'Teen' just in case.