The Halacha of "Tum’at Ohel" establishes that everybody under the same roof as a human corpse becomes Tameh. For this reason, it is forbidden for a Kohen to come under the same roof as a dead body, because he will then become Tameh.
Is it permissible for a Kohen to be in the same room as a body part separated from a living person? Does the prohibition apply only to corpses, or also to parts of the body of a living person?
The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) addresses this issue in Hilchot Tum’at Met (2:3), where he writes that a body part from a live person does generate Tum’a under certain conditions. Namely, if it is a complete organ and has flesh, tendons and bones, then it brings Tum’a to everybody in the room. If, however, there is only some flesh without bones, then there is no Tum’a and it would be entirely permissible for a Kohen to enter the room. Thus, for example, a Kohen may be in the same room as part of a tongue from a live person, or the intestines. This applies regardless of the amount of flesh in the room.
This Halacha is very relevant to Kohanim who work as doctors and perform surgical procedures involving the removal of body parts from live patients. Before performing such a procedure, they must ascertain whether the body part in question meets the conditions to generate Tum’at Met. It should be noted that a Kohen may perform a biopsy, as the amount that is removed from the patient is smaller than a Ke’zayit and thus does not generate Tum’a, besides the fact that there are no bones in the parts that are removed.
Summary: A Kohen may be in the same room as body parts taken from a live person, as long as it is not a complete organ with bones, tendons and flesh. Thus, for example, a Kohen who is a doctor may perform biopsies and other procedures where small amounts of flesh are removed from the patient.
