However, in this week's parshah, there is a whole other dimension. It says in a certain book that, if you calculate 24 hours over forty days, the total is 960 hours. The point of a mikvah is to purify either people or things. Spiritual impurity is not like a virus, or germs in general, which may be invisible to the eye without a microscope, but are nevertheless physical. Spiritual impurity is not physical at all, though it can end up having direct physical effects.
Spiritual impurity can come from many physical sources, such as a dead body.
The less spiritual something is, the more chitzoni it is. This aspect of impurity represents three levels of sources of spiritual defilement, which makes the total 960. Hence, Moshe's stay on Mt. Sinai acted as a sort of mikvah for him. It separated him from the world of chitzonios, and made him into a fitting container for the light of Torah he was trying to receive. In fact, this might explain the Midrash that says that every day Moshe kept forgetting the Torah G-d taught him.