Gemara: "… No nega (affliction) will approach your tent" (Tehillim 91:10) – that you should not find your wife to be a safek (doubt of being a) nidda when you come back from a trip.

Ein Ayah: A nega (the Torah uses it also in regard to leprosy) is an external affliction that is not along the lines of the normal functioning of the body.
The phenomenon of a person being prevented from receiving the enjoyment of a natural desire is intended so that he can receive a positive gain from it. If it is difficult for him to give up something, but he agrees to it because the Torah requires him to do so, he will gain doubly. He will strengthen his ability to control himself and adorn himself with ethics and fear of Heaven.
There are two categories of cases where a person has to hold himself back due to the Torah. Generally, he must hold himself back because of that which is good in essence, as the Torah demands one to make the necessary efforts to keep himself on a level of sanctity and spiritual elevation. However, there are also situations where, if a person maintained the proper level of sanctity and carefulness in following the mitzvot properly, he would not need additional cases of holding himself back. Only after falling from his level, he needs additional restrictions, which serve as a bitter remedy for his malady. While that may be the proper thing under the circumstances, it would have been better not to have gotten sick in the first place. It is like an external lesion: while it cleans the body of the bad internal infection, it would have been better had the infection not existed in the first place.
The above is a parable to a case of something which is forbidden out of doubt. The doubt often comes from lack of knowledge, lack of carefulness, or lack of wisdom. It would have been better if the person could decide the matter in a clear-cut fashion, but under the situation of uncertainty, it is a helpful remedy to suffer with love of Hashem the obligation of having to refrain out of doubt. It is a blessing not to need these "restrictions of chance," which are a sign that he has not reached completeness, but to only need to refrain from things that the Torah determined are intrinsically proper to refrain from.
This phenomenon also applies to Israel as a nation. When we live in Hashem’s Land and all the matters of the nation are in place, with Sanhedrin seated in the "place that Hashem chose," there are no doubts in matters of implementation of the Torah. In such circumstances, whenever there is a matter that is beyond the intellectual grasp of individuals, the Torah commands to "do according to the matter they will tell you from that place" (Devarim 17:10). However, exile and dispersion have caused many doubts. We must accept the situation with great love, for in our state of disease, this is specifically our remedy. Yet, we should look forward to a time of "from Zion shall Torah go forth" (see Yeshaya 2:3) and the return of our judges to the way it was in days of yore (see Yeshaya 1:26). Then our Torah-based behavior will certainly be in a manner that makes the heart and spirit happy, and the nation will be led by the Torah in the manner that was originally and intrinsically intended. Then we will apply the pasuk: "All your sons will be learned in the ways of Hashem, and there will be much peace for your sons" (Yeshaya 54:13).