Tum’at Met – the status of impurity emitted by a human corpse – has a number of unique stringencies which do not apply to other forms of Tum’a (impurity). For example, the Torah establishes in Parashat Hukat (Bamidbar 19:14) that a corpse emits Tum’a "Be’ohel" ("in a tent"), which means that even being under the same roof as a corpse, without direct contact, brings Tum’a onto a person (or utensil). Additionally, the only way to rid oneself of Tum’at Met is through the sprinkling of the special waters mixed with the ashes of the Para Aduma (red heifer).
Another unique feature of Tum’at Met is indicated by the Torah (Bamidbar 19:16) when it speaks of somebody touching "Halal Hereb" – the corpse of somebody killed by a sword. The laws of Tum’at Met apply to a corpse regardless of how the person died, and yet the Torah found it necessary to specify the particular case of a person killed by the sword. The Sages explained that the Torah mentioned the case of a "Halal Hereb" to instruct that "Hereb Hareh Hu Ke’halal" – the sword that killed somebody has the same Halachic status as the corpse. This means that a metal utensil which touched a corpse emits Tum’a like the corpse itself.
Tosafot (Talmud commentaries by Medieval French and German scholars) in Masechet Nazir (54b) relate that Rav Haim Kohen heard what he regarded as an astonishing Halachic ruling by Rabbenu Tam (France, 1100-1171) relevant to this law. Rabbenu Tam reportedly ruled that since a metal utensil which came in contact with a corpse has the same status as a corpse, it emits Tum’a "Be’ohel," bringing impurity upon all people and objects in the building. This ruling results in an extraordinary stringency. It means that if a person attended a funeral while wearing a watch, such that the watch becomes Tameh (impure), that person then brings Tum’a to people and utensils whenever he walks into a building wearing that watch.
Rav Haim Kohen – who was, of course, a Kohen – reacted to this ruling by citing G-d’s exclamation in the Book of Yeshayahu (66:1), "Ezeh Bayit Asher Tibnu Li" – "What house can you build for Me?!" He said that as a Kohen, who is forbidden from contracting Tum’at Met, he cannot enter any house according to Rabbenu Tam’s ruling. After all, virtually every house must have a utensil that at one point had come in contact with a human corpse, and thus if Rabbenu Tam is correct, Kohanim cannot enter any building, which is, quite obviously, an untenable conclusion.
Rabbenu Bahya (Spain, 1255-1340) brings this exchange in his commentary to Parashat Hukat, and notes that the accepted Halacha does not follow Rabbenu Tam’s opinion, and utensils which had come in contact with a corpse emit impurity only through direct contact, and not "Be’ohel."
Moreover, the Rama (Rav Moshe Isserles of Cracow, 1530-1572) writes that although a metal utensil that had come in contact with a corpse conveys Tum’a, it is not forbidden for a Kohen to come in contact with such a utensil at all. Although the Kohen becomes Tameh by touching this object, this is not forbidden, as the prohibition forbids only coming in contact with a corpse itself.
Another significant leniency noted by the Rama is that while a Kohen is in contact with Tum’a, he does not violate a prohibition by coming in contact with another source of Tum’a at the same time. Thus, if a Kohen, for whatever reason, is touching a corpse, he is allowed to touch another corpse at the same time, since he is already "defiled" by virtue of his touching the first corpse, such that touching the second corpse is inconsequential.
These two lenient rulings of the Rama led some to devise a creative – though very questionable – solution for Kohanim who wish to study and practice medicine, which generally requires them to come in contact with human corpses (such as by dissecting cadavers as part of their training.) The proposed solution was to have a ring touch a corpse, and then have the Kohen wear the ring whenever he comes in contact with a corpse. As we saw, a Kohen is permitted to touch this ring, even though it causes him to become Tameh, and, while he touches one source of Tum’a, he is then allowed to touch another source of Tum’a. Therefore, some thought that this might solve the Halachic problems that arise when a Kohen wishes to study and practice medicine.
The consensus among the Poskim, however, dismisses this solution, noting that the Gemara never raises such a possibility. In Masechet Erubin, the Gemara discusses the case of a Kohen who had his Erub Tehumin placed in a cemetery, in order to extend the distance that he may walk on Shabbat, and the Gemara rules that such an Erub is invalid, because it cannot be accessed by the Kohen. According to the line of reasoning presented above, the Kohen could, conceivably, access it by putting on a ring which had come in contact with a corpse, and then entering the cemetery while wearing the ring. The Gemara did not conceive of such a solution – quite likely because this solution is not Halachically sound. Likewise, the Gemara states that if a Kohen instructs his son to bring him something from a cemetery, the son must refuse his father’s instruction, since this entails a Torah violation. The Gemara does not propose the solution of the son wearing a ring which had been in contact with a corpse so he may then enter the cemetery.
The likely reason why this solution is not valid is because a Kohen touching a source of Tum’a is permitted to touch a second source of Tum’a only if he touches the first source of Tum’a in a prohibited manner. If he is touching a permissible source of Tum’a, then he is not "defiled" such that no further defilement occurs by touching a second source. It is only when he touches a prohibited source of Tum’a that he becomes "defiled" and thus is allowed to touch a second source of Tum’a at the same time.
Therefore, this solution has not been accepted by the Poskim, and a Kohen who wishes to pursue a career in medicine must consult with his Rabbi for guidance.
Summary: A Kohen is forbidden from touching a human corpse, and also from being under the same roof as a human corpse, and therefore, a Kohen who wishes to pursue a career in medicine must consult with his Rabbi for guidance. A metal utensil – such as a watch or ring – which comes in contact with a human corpse emits Tum’a (impurity) like a corpse itself, but a Kohen is nevertheless allowed to touch such a utensil.