In Exodus 15:26 God enters into a covenant or a contract with the Israelites. A covenant is an agreement, contract, promise or a pact between two people or two parties. In this covenant God tells the Israelites, if you obey my commands, you will be free from sickness because I am your healer.

Deuteronomy 28 is a chapter that outlines the blessings for obedience to God’s commands and the curses for disobedience to God’s commands. As you will see below, sickness is a curse.

The Lord will send upon you curses, confusion, and rebuke in all you undertake to do, until you are destroyed and until you perish quickly, on account of the evil of your deeds, because you have forsaken Me. The Lord will make the pestilence cling to you until He has consumed you from the land where you are entering to possess it. The Lord will smite you with consumption and with fever and with inflammation and with fiery heat and with the sword and with blight and with mildew, and they will pursue you until you perish.

Deuteronomy 28:20-22

If you are not careful to observe all the words of this law which are written in this book, to fear this honored and awesome name, the Lord your God, then the Lord will bring extraordinary plagues on you and your descendants, even severe and lasting plagues, and miserable and chronic sicknesses. He will bring back on you all the diseases of Egypt of which you were afraid, and they will cling to you. Also every sickness and every plague which, not written in the book of the law, the Lord will bring on you until you are destroyed.

Deuteronomy 28:58-62

Basically, God is saying in the above verses, if you break my commandments, you break my covenant I made with you in Exodus 15:26 and I will allow sickness to come upon you. The Israelites were a very disobedient bunch. They were always fault finding, complaining and grumbling about something. However, when they repented of their sins, they were healed. Numbers 21:4-9 below is a good example of how the covenant between God and the Israelites works. The Israelites became disobedient, then they repented and God healed them.