There is an machlokes (Tanaim) if there is an issur dirabanan (see Ohalaos Ch. 3) to import vegetables from chutz la'aretz to Eretz Yisrael because we are concerned that there will be some dirt on the vegetables and the dirt of chutz laaretz is tamei because we're worried there are human remains mixed with the dirt. See Sanhedrin 12a and Nedarim 53b. Tosfos (Sanhedrin) says the problem is that one may touch the dirt and become tamei. Rashi (ibid.), however, says the problem is that one will become tamei when they will be under the same ohel as the vegetable and will become tamei from ohel. Rashi apparently holds like the strict opinion (Rabbeinu Tam) - that a non-Jewish corpse is metamei b'ohel. (While this is only tamei midirabanan, Rashi apparently held the chachamim made it tamei to the same extent as it would be had it been dioraissa - even in an ohel.) It also seems Rashi holds this way in Ramban Yevamos 61a. I just want to remind everyone that not all rishonim agree with Rabbeinu Tam (see Ramban Yevamos 61a s.v. ha, and Nimukei Yosef Bava Metzia 69b bedapei haRif s.v. ein), and the Shulchan Aruch paskens like Rabbeinu Tam lechumra - "it is correct for kohanim to not go..."
About the machlokes if one may import vegetables - I'd like to add one point. The problem of the vegetables is that they grow in the ground, and some dirt may be attached to them. This wouldn't apply to grain or fruit, which grows higher off the ground and would not have dirt sticking to it. It seems to me that there is a hint to this halacha in a pasuk in Eishes Chayil (Mishlei 31:14) haysa ka'anios socher, mimirchak tavi lachmah, she was like a merchant's ship, from far-away places she brings her bread. Shlomo Hamelech is alluding to this halacha that bread (wheat) may be imported from far away lands, but vegetables may not be imported.
Another point that only Yisrael is called Adam, and adam is plural as well as singular - perhaps this is a reason why it is forbidden to count Jews 1, 2, 3, ..., instead we count their shkalim (or we say "hoshia es amech" to count a minyan) - to reduce Jews to individuals we negate the unity aspect - the Adam - of Klal Yisrael.