ויקרא, פרק י״א, פסוק כ״ח

The laws of purity and impurity draw a fundamental line between life and death. Animals cannot become impure, nor can they generate impurity, as long as they are alive. It is only upon death that a creature transforms into a primary source of impurity [רש ר הירש]. When a person moves an animal carcass from one place to another, they contract impurity. The primary approach among commentators is that this transfer of impurity occurs through the sheer act of moving the carcass, even if the person never touches it directly with their hands [רשב״ם, בכור שור, חזקוני, הכתב והקבלה, ביאור יש״ר].

However, the transmission of impurity through carrying requires actual physical movement. If a carcass is placed upon a person's shoulder, they remain completely pure as long as they do not touch it and do not move from their spot [רשב״ם, הכתב והקבלה, ביאור יש״ר]. The guiding principle is that impurity falls upon the one actively carrying the carcass, but not upon someone who is merely being carried alongside it [אדרת אליהו].

The act of carrying a carcass affects more than just the physical body. A person's clothes also become impure, regardless of their size or color. This principle extends to most other everyday items and vessels, which contract impurity in the exact same manner as clothing. There are, however, exceptions to this rule: earthenware vessels and other human beings do not become impure through this specific process [אדרת אליהו].

The laws governing both touching and carrying an impure carcass work together as a unified system [הטור הארוך, ביאור יש״ר]. Within this framework, different pieces of an impure animal carcass can combine to reach the minimum physical amount required to transfer impurity. Yet, these pieces can only join together with parts from a similar source; they cannot combine with entirely different sources of impurity, such as a human corpse or a creeping creature [אדרת אליהו].