When getting a vaccination, which substance is injected into the body

Interestingly, the immune system becomes stronger when it is fighting a disease that it has already encountered before. So during vaccination, the body’s immune system is injected with the certain type of virus or bacteria in order to introduce the body to the disease and help the system combat it and get stronger when battling against the real one. For example, in order to gain immunity to small pox, a vial containing the disease is injected to a child so that he cannot catch the disease in the future.

If you remove all the lions from the savanna it will lead to a increase of gazelle and a increase of cheetahs becasue of no competition