As we learned on yesterday’s daf, although a nazir cannot allow himself to become tameh met, not all situations of tum’at met will force him to begin his nezirut over again. According to the Mishna (54a-b) there are many cases where the nazir may formally become tameh met, but he will not need to begin his nezirut over again, rather he will have to wait a week and undergo the process of taharah – purification – after which he will be allowed to resume his nezirut at the point where it was interrupted. Similarly, he will not have to shave his head like a nazir tameh.
One such case is tum’at eretz ha-amim – the ritual defilement of foreign lands. There are two suggestions made by the Gemara to explain this enactment of the Sages. Tum’at eretz ha-amim is either mishum avira – “because of its air” – or mishum gusha – “because of its earth.” Rashi explains these positions as technical statements. The Gemara is asking whether a person must step on the ground outside of the land of Israel to become tameh, or whether even traveling through its air would be enough to subject the individual to rabbinic tum’ah.
The Rosh takes a different approach to explaining this law. According to the Rosh, saying that the air of foreign countries is the source of tum’ah essentially means that the reason for the rabbinic enactment does not stem from a fear that there are dead bodies there, rather it is an independent decree whose purpose is to discourage Jews from living outside the land of Israel. According to this approach, the idea that tum’at eretz ha-amim is mishum gusha means that outside of Israel we are concerned that there are bodies buried in places that we do not know about, so we must always assume that there is safek tum’ah – the possibility of tum’ah – wherever one goes.