infiltrate

Concise Oxford English Dictionary © 2008 Oxford University Press:
infiltrate /ˈɪnfɪltreɪt/

▶verb

• 1 surreptitiously and gradually enter or gain access to (an organization, place, etc.).

• 2 permeate by filtration.

▶noun Medicine an infiltrating substance or group of cells.

– derivatives
infiltration noun,
infiltrator noun.

– origin ME (as infiltration): from in-2 + filtrate.