The 96th mitzvah is that we are commanded regarding the tumah of neveilah. This mitzvah includes tumas neveilah and all of its laws.1
I will now give an introduction you should remember all through the following discussion of the various types of tumah. When we count each category as a separate positive commandment, this does not mean that one is commanded to become tameh with this form of tumah; nor that one is forbidden from becoming tameh with it, as if it is a prohibition. Rather, the Torah's statement that anyone who touches a certain category is tameh, or that a certain category conveys tumah in a certain way to one who comes in contact with it — this itself is the positive commandment. This means that the law we are commanded to follow — that if one touches a certain object in a certain way he becomes tameh, but in another way, he does not — is itself the mitzvah.
The actual decision to become tameh or not, however, is optional. If one wants, one is allowed to become tameh, and if one doesn't want, he doesn't have to. The Sifra says, "From the verse,2 'Do not touch their neveilah,' I might think that one who touches neveilah receives 40 lashes — the Torah therefore says,3 'To these you will become tameh.' I might think that if one sees neveilah he must go and become tameh from it — the Torah therefore says, 'Do not touch their neveilah.' How is this possible? It is optional."
Therefore the mitzvah is the actual ruling regarding these laws, that we are commanded that anyone who touches a certain object becomes tameh and is bound by all the obligations of those who are tameh — to go outside the machaneh Shechinah, not to eat or touch sacrificial food, etc. This itself — that he becomes tameh through touching or some other connection4 — is the mitzvah.
You should remember this principle throughout all the categories of tumah.
