Ex. 15:26 is an integral verse to the Old Testament, perhaps the key passage to understanding everything that comes later. In the KJV it reads, "If thou wilt diligently hearken to the voice of the LORD thy God, and wilt do that which is right in his sight, and wilt give ear to his commandments, and keep all his statues, I will put none of these diseases upon thee, which I have brought upon the Egyptians: for I am the LORD that healeth thee."
Looking at the later history of the Jews, it is clear God keeps his word. When the Hebrews follow the commandments, they live well. As soon as they break them, God punishes them swiftly and severely. At least they can't say they weren't warned.