No Diseases of Egypt
The Vilna Gaon has an interesting explanation as to why the Passover Hagaddah bothers to tell us of a three way argument regarding the number of plagues that smote the Egyptians. There were opinions of fifty, 200, and even 250.
His answer is related to the promise made by Moshe Rabbeinu. He told the people that all of the diseases of Egypt, (מדוי מצרים) will not be placed on you. All that needs to be done is to diligently observe the commandments.
The point of the Gaon was that if more diseases were delineated, the less the chance, that these same diseases would befall the Jewish people.
As long as they followed the Torah, none of the two hundred and fifty Egyptian diseases would smite. This would minimize the potential for sickness among the Jewish people.
The Torah has previously mentioned that Egypt was the most decadent place on earth. Not only was it filled with physical diseases, but it was equally plagued with moral disease. The Rabbis were hinting to the fact that it was their hope that none of the moral or physical diseases of Egypt, would ever fall on the Jewish people.
We live in a world of moral depravity as well as one of numerous diseases. The Jewish people must build an immunity to all of this, and stay on the path designated for them by the Torah.