Food and drink become impure when they come in contact with a source of impurity or with a person or utensil that has become impure through such contact, as it says "[And anything that part of them falls on...] any food that is eaten that water has come on becomes impure and any drink that is drunk in any vessel becomes impure".¹ Certain types of sacred things that have become unfit are also treated as though they had been in contact with a source of impurity. Rabbinically, when impure food touches other food it becomes impure to a lower degree; but when a fluid becomes impure it is as if it had touched a source of impurity. Contact with an impure fluid even makes utensils impure unless the fluid became impure by contact with impure hands or with one who has eaten impure food or washed in "drawn water".^a If a person who has bathed for his impurity on that day touches food or drink, it acquires the lowest degree of impurity.

In ordinary food there are only two degrees of impurity (food that touches a source of impurity and food that touches such food), as it says "And if any of them falls into an earthenware vessel everything that is in it becomes impure".² Thus hands that become impure, Biblical writings, or a person who has eaten impure food or washed in "drawn water" do not make ordinary food impure (but do make fluids impure). In heave-offerings there is a third degree of impurity (so that all of these things do make them impure), as it says "[He shall be unclean until evening and shall not eat of the sacred things unless he washes his flesh in water] and the sun shall set, and he shall [then] become pure and afterwards he shall eat of the sacred things, for it is his bread".³ In sacred things there is a fourth degree, as it says "And the flesh that touches any impure thing shall not be eaten".⁴ All non-sacred meat is regarded as being impure in the third degree.^b

Food cannot become impure until after it has been in intentional contact with water, dew, oil, wine, milk, blood or honey, as it says "[Any food... that water has come on...] and if water is put on produce and part of their corpse falls on it, it is unclean for you"⁵; and only these seven fluids can become impure. Food cannot become impure until it is detached from the ground or the animal; an animal cannot become impure until it has died or been slaughtered; and water that is on the ground cannot become impure without special intent. Food and drink cannot become impure unless they are fit (and in the case of food, intended or normally used) for human consumption, as it says "[Any food that is eaten...] and any drink that is drunk".^l,c

Sources:
1. Lev. 11:34	a. She'ar Avos ha-Tumos 7:1,2,5; 8:3,10; 9:1; 10:3,7-10
2. Lev. 11:33	b. She'ar Avos ha-Tumos 11:2-5,7-8; see 13-15
3. Lev. 22:6-7	c. 1:1-2,4; 2:1,4,14,18; 12:1; 15:1,5
4. Lev. 7:19
5. Lev. 11:34,38