A impure person or utensil (except for earthenware or glass) becomes pure by immersion in [a sufficient quantity of] water that is on the ground, as it says ["And he shall wash all his flesh in water and be impure until evening",¹ and it says] "It shall be put into water and shall be impure until evening and become pure".² (Impure water also becomes pure by contact with such water; but impure food can never become pure unless it becomes unfit for a dog to eat.) The entire person or utensil must be immersed at one time and there must be no significant obstacle that prevents contact between him or it and the water.^a

Impure hands become pure either by immersion or by pouring water on them from a container; in either case there must be nothing that prevents contact with the water and all of the hand must be wet at the same time.^b

Rabbinically, the immersion must not be in "drawn water", as it says "But a spring or cistern gathering place (mikveh) of water shall be pure"³ -- just as a spring is not man-made, so the water in the cistern cannot be water drawn by man in a container. A man impure because of a running issue can be purified only by immersion in a spring, as it says "[And he shall wash his flesh] in living water [and be clean]".^4,c

It is permissible for a person to become impure; a priest and a nazirite are forbidden to come in contact with a corpse, but even they are permitted contact with all other types of impurity. All of the laws regarding purity and impurity are only on account of sacred things; an impure person is forbidden to enter the Temple or to eat sacred food and offerings, but there is no prohibition as regards the non-sacred. Contact with the impure is permissible, as it says "Pure and impure together".⁵ Nevertheless, it is proper for a pious person to avoid such contact since separation leads to holiness, as it says "And you shall make yourselves holy and be holy, for I am holy".^6,d

Sources:
1. Lev. 15:16	a. 1:1-3,7,12; Tumas Ochlin 2:18,21; see Kelim 26:14
2. Lev. 11:32	b. 11:1-2,7; see Berachos 6
3. Lev. 11:36	c. 1:5; 4:2; 6:1-2; 9:8
4. Lev. 15:13	d. Tumas Ochlin 16:8-12
5. Deut. 12:22
6. Lev. 11:44