Jesus gave his disciples authority in the spiritual realm:

«And when He had called His twelve disciples to Him, He gave them power over unclean spirits, to cast them out, and to heal all kinds of sickness and disease».

Matt 10:1

In the original language, it means authority over an enemy and the power to cast him out of sick bodies. This was something completely new in the history of the Bible: Authority over evil spirits and diseases.

Just as a policeman has authority to arrest a lawbreaker, the believer can arrest the sickness in Jesus' name and cast it out of the sick person's body. Behind the policeman's authority stands the Parliament and government that made the laws the police must adhere to. We talk about delegated authority. Be "God's police" in a world with much disease and suffering. Take authority over sickness. Arrest them in Jesus' name and cast them out of sick bodies. 

Jesus said:

«In My name they will cast out demons... They will lay hands on the sick, and they will recover.»

Mark 16:17–18

To do it in Jesus' name means that you act on His behalf, according to His riches, orders, and will. The emphasis is not on you, but on Him who gave the command. And He is «God Almighty» (Gen 17:1). This title is used 218 times in the Bible about God. It is not just anyone who has sent us out into the world!