infiltrate >> ˈɪnfɪltreɪt
Definition: [verb] enter or gain access to (an organization, place, etc.) surreptitiously and gradually, especially in order to acquire secret information.
Example: the organization has been infiltrated by informers
Definition: [verb] (of a liquid) permeate (something) by filtration.
Example: virtually no water infiltrates deserts such as the Sahara
Definition: [noun] an infiltrating substance or a number of infiltrating cells.
Example: a chest radiograph revealed a patchy infiltrate in the left lower lobes