Whoever touches the dead body of any person shall be unclean seven days.
Numbers 19:11

Contact with death or things closely associated with death makes a person ritually unclean. Being in the presence of death drains life from the living and touching a corpse, human bone, or grave and being in a building when a person dies make a person unclean for seven days.
Numbers 19:14-16

Clean and unclean are roughly analogous to life-adding and life-subtracting. They don't really have anything to do with being physically clean and dirty.
#chukat #torah