Do you think that has always been the case, or that it changed at some point? If Israel were obedient, the LORD would put upon them none of the sicknesses that He put upon the Egyptians. Is there any reason to think that Satan had anything directly to do with the sicknesses the Lord put upon the Egyptians? God took credit for the plagues, which isn't to say that He couldn't use angels. One of the Herods was struck by the angel of the Lord. What about Jacob's injury to his thigh after his all-night wrestling match?