Why

"Um, Scoot?" whispered Rainbow Dash when she returned home from the funeral of Twilight.

"Well, Rainbow, it's nothing... it's just a matter of life. Even my other crusaders know it." replied Scootaloo calmly. She thought that Rainbow was still sad from the loss of her friend.

But it was not the death of Twi that Rainbow was sad. It was that she feared for herself that she will die. The Yellow Disease was sweeping across Equestria like wildfire, no matter what the hospitals did to save their patients. It is just too strong.

"...Maybe...I...won't teach you how to fly straight in a loop today. I...have something else to do." Rainbow stammered.

Scootaloo felt strange towards Rainbow. She was usually so bright and bubbly, always up to a horror story at night. But now, her stories are getting shorter and cheesier, even though the time was only 7pm. There's something seriously wrong with her, thought Scoot.

And she was right. Due to her continuous loss of sleep and depression, Rainbow contracted the Yellow Disease. The yellow bits in her mane and cutie mark spread quickly, covering her whole body. The funeral of her came a few days later.

"Guys, this is serious. Two out of six Elements disappeared. We have to do something, quick! Since the Elements are no longer complete and they could not use them to wipe out the plague, it's up to us fillies to do it." Scoot said on the day of RD's funeral.

"But what Scoot? We don't have a speck of an idea in our heads right now!" Apple Bloom blurted out.

"Good question, Bloom. Come back tomorrow at the clubhouse to see if you have thought of anythin'." Scootaloo replied. Then, the three of them dispersed, each with a wondering mind.