History of onions and garlic
(The following story was kindly provided by HH Bhaki Purusottama Swami)
One ayurvedic doctor had told me that this description is there in Ayurveda: When Lord Vishnu in His Mohini form was distributing nectar to demigods two demons named Rahu and Ketu sat down along with the line of Demigods. By mistake the Lord served them nectar into their mouths. Immediately the lord was informed by the Sun and Moon that those two were demons.
As soon as the Lord came to know of this He cut off the heads of both demons. By that time nectar had not passed through their throats. It was still in their mouths. When the Lord cut of their heads the heads were separated from their bodies. Thus nectar did not pass to the stomach but fell on the ground. (That is the reason why Rahu and Ketu's head are still alive but their bodies were finished)
When their heads were cut off garlic and onions manifested from this nectar which fell on the ground from the mouth of the two demons. Thus Garlic and Onions are regarded as nectar but not nectar touched the mouths of these two demons still garlic and onions act like nectar in curing the diseases. But they are not meant for Vishnu or Vaishnavas.
The doctor told me that whoever will eat garlic and onions, their body will be very strong like demons body and at the same time their intelligence also will be contaminated like the intelligence of demons.