Numbers 19:11-13 explain the general rules for the use of this water. Numbers 19:14-22 give a more specific description of the application of the general rules. Numbers 19:17-19 record the ceremony of purification. [source][source][source]
"Animals are clean and unclean, not because they necessarily will or will not make a person sick if they are used as food; they are clean and unclean primarily because God desired his people to live in a world of discrimination (see esp. Leviticus 11:44-47). We may look back from a twentieth-century understanding of infection and disease and remark, "How kind it was of God that some of the animals he declared to be unclean to Israel are foods that might be conveyers of disease." But the principal issue is distinction, discrimination, the marking out of that which is different from something else." [1][source]
Nonetheless we should not discount God"s care for His people"s physical welfare even though that may have been His secondary reason for legislating as He did. [source][source][source]
"God recognized that the incubation period for most bacteria is within seven days. This means that after exposure to a disease, a person will know within seven days whether the disease is contracted.... [source][source][source]
". . . the "unclean" provision of seven days was practical for most acute, bacterial diseases fatal in that day. [source][source][source]
"Hand washing and clothes washing with proper drying were prescribed in Numbers 19:19 ... Numbers 19:21 notes that "anyone who touches the water of cleansing will be unclean till evening." These provisions recognize that not only is washing important in mechanically cleansing one from microbes, but drying ("until evening") is also essential. Pathogenic microbes can live in moisture that remains on skin, dying when the skin is eventually dried. Furthermore Numbers 19:13; Numbers 19:18-21 refers to the provision of "sprinkling" the water, which indicates the need for running water, not stagnant water. Again this is a more effective means of cleansing, though more cumbersome. [source][source][source]
"Did the average Israelite understand the significance of this preventive medical standard God imposed? No doubt he did not. However, God knew and in His wisdom cared for His people." [2][source]
This sacrifice, then, was a kind of instant sin offering and provided for the cleansing of those who had become ceremonially unclean through contact with a corpse. The unclean person who refused to purify himself would suffer death ( Numbers 19:13; Numbers 19:20). To refuse cleansing was to repudiate the divine revelation concerning the relationship of sin and death. This sacrifice kept the Israelites free from the defilement that would hinder their fellowship with God (cf. 1 John 1:7-9; Hebrews 9:13-14). Jacob Milgrom believed this offering was thought to exorcize a demon that came with corpse contamination. [3][source]
"This chapter provides an alternative remedy which marked the seriousness of the pollution caused by death, yet dealt with it without the cost and inconvenience of sacrifice. Instead, those who have come in contact with the dead can be treated with a concoction of water that contains all the ingredients of a sin offering." [4][source]
"The writer"s concern for the ritual of the red heifer at this point in the narrative ... finds its roots in the earliest narratives of Genesis where death itself is viewed as the ultimate defilement of God"s good creation. As such his point appears to be to show that just as in the beginning, so now among God"s covenant people, death is the arch enemy." [5][source]
This sacrifice is similar to the sacrifice of Christ that cleanses the Christian from the defilement that we contract as we live in the world ( 1 John 1:9). [source][source][source]