Torah Portion: Shemini
“…וכי ימות מן הבהמה אשר היא לכם לאוכלה…” (ויקרא יא לט)
It is a positive commandment that a carcass be impure and that it imparts impurity [to other items] — as it is written: If an animal that you may eat has died, one who touches its carcass shall become contaminated until evening (Vayikra 11:39).
The Rambam writes about this mitzvah that the fact that we count this concept of impurity as a positive commandment does not mean that there is a mitzvah to become impure or a negative commandment that we not become impure. Rather, the idea that the Torah says that one who comes into contact with this type of item [i.e. a carcass] becomes impure — that in itself is the positive commandment. But the action of becoming impure is given to the discretion of every person; if one wishes he may become impure, or, if he wishes, he may refrain from becoming impure.