Patanjali Yoga Sutras Knowledge Sheet 1
We will begin with a story, the greatest and most effective way of conveying knowledge. Once upon a time, long ago, all the munis and rishis approached Lord Vishnu to tell him that even though He (incarnated as Lord Dhanvanthari) had given them the means to cure illnesses through Ayurveda, people still fell ill. They also wanted to know what to do when people got sick.
from Yoga Buzz http://www.artofliving.org/yoga/patanjali-yogasutra/knowledge-sheet-1