This kind can come out by nothing but prayer and fasting.

Mark 9:29

The greek word for kind in this verse is genos: offspring, family, race, species. From this we get the english words gene, genome, genetic and so on.

In the gospels many of the physical ailments people suffered from were associated with spiritual causes—Matthew 4:24, 10:1; Luke 6:18, 9:1, 13:11, etc. Jesus forgave the sins of the paralytic before healing him, loosed the woman from her spiritual infirmity before straightening her back. The emphasis New Testament writers placed on the spiritual roots of these negative feedback loops seems largely ignored today.

In the mid 19th century Gregor Mendel, growing peas in a monastery, discovered genetics—realizing it takes three to four generations for a dominant gene trait to become recessive, and bringing into sharper focus the physical realities of the spiritual law Yahweh revealed to Moses in Exodus 34:7.

Since then, we have discovered genes that cause breast cancer (BCR1), Down syndrome, and murderous behavior (MAOA).

In the most literal translation of this verse, Jesus is talking about a species of unpure spirit. He may, however, also be referring to a genetic defect (epilepsy).

Either one or both give us something to pray and fast about.