Professor V.P Tokin of Leningrad University in 1930 discovered the fact that smelly substances secreted by garlic, onion and pine trees kill single cell organisms like ameba, typhoid, dysentery, and tuberculosis germs and named the substance Phytoncide, and used it ever since.

If so, why plants secret such a unique substance?

Rooted trees can't move, even when they are attacked by enemies. So, in order to protect themselves, trees manufacture protective substances own its own.
In this manner, general term for every antibiotic substances that plants trees release in the air or on the ground directionally is called Phytoncide.

It can be said that what we call 'forest energy' refers to Phytoncide, and forest walking can be summarized as humans coming into contact with vaporized Phytoncide in the atmosphere.

Public's spotlight on Phytocide is for the fact that it acts as killer for various insects, germs, molds and bacteria which threatens plants, but in contrast it's beneficial to humans.
Phytoncide is not a chemical compound but a natural substance. It is quickly absorbed by human body without any difficulty and it destroys only harmful germs selectively.