ויקרא, פרק י״א, פסוק ל״ט

After detailing the laws regarding inherently impure creatures, the focus shifts to the conditions under which pure, permitted animals can transmit impurity. A fundamental distinction exists between the two categories. An impure animal automatically becomes a source of impurity upon death, regardless of how it dies. In contrast, an animal belonging to a species naturally permitted for food [אבן עזרא, שטיינזלץ] only becomes impure if it dies of natural causes or undergoes an invalid slaughter [רשב״ם, חזקוני, רלב״ג, ביאור יש״ר, רש״ר הירש, הופמן, העמק דבר].

Proper slaughter possesses a unique ability to prevent impurity, even when the animal ultimately remains forbidden to eat. For instance, if a pure animal suffers a fatal defect but is still slaughtered correctly, it does not convey impurity. However, for slaughter to be effective, the animal must be a viable, sustainable life. If an underdeveloped eight-month-old fetus is born alive and slaughtered, the procedure does not prevent impurity, because the creature is not considered naturally ready for consumption [תורה תמימה, מלבי״ם, אדרת אליהו]. Furthermore, the status of an unborn fetus is entirely dependent on its mother. Just as a fetus inside a pure animal is made permissible by the mother's valid slaughter, a fetus inside an impure animal does not generate a separate state of impurity distinct from its mother [תורה תמימה, אדרת אליהו].

Impurity is not limited to an intact, whole corpse. It applies to partial remains, including a severed limb taken from either a living or dead animal, provided the limb is complete with flesh, sinews, and bones [תורה תמימה, מלבי״ם, רש״ר הירש]. The physical state of the remains also matters; the flesh must retain the natural moisture it had at the time of death, rather than being completely dried out [תורה תמימה]. The capacity to transmit impurity also extends to parts of the animal that serve a functional human need, such as natural "handles" used to grip the meat [תורה תמימה, מלבי״ם].

Physical contact with a carcass conveys impurity, provided the remains are at least the size of an olive [רלב״ג, פרדס יוסף]. Yet, not all anatomical parts transmit impurity independently. Bones, sinews, horns, hooves, hair, and skin are exempt on their own [רש״י, מזרחי, רבנו בחיי, רש״ר הירש]. However, if these elements remain attached to the flesh, they act as protectors or handles for the meat, meaning that touching them is treated exactly like touching the flesh itself [מזרחי, שפתי חכמים, משכיל לדוד, דברי דוד]. Similarly, a completely sealed hollow bone does not transmit impurity because the marrow inside is inaccessible. If the bone sustains even a microscopic puncture, the marrow becomes exposed, and the bone conveys impurity [תורה תמימה, מלבי״ם].

A person contracts this impurity not only through direct physical touch but also by carrying the weight of the carcass [רלב״ג, רש״ר הירש, פרדס יוסף]. This transmission is strictly limited to external contact or bearing a load; impurity that is entirely swallowed and contained within the human digestive tract does not trigger these laws [רש״ר הירש, פרדס יוסף]. Once a person becomes impure through contact or carrying, the state lasts until evening and requires washing in water to achieve full purification [אבן עזרא].