1 [intransitive always + adverb/preposition, transitive]
to secretly join an organization or enter a place in order to find out information about it or harm it:
Police attempts to infiltrate neo-Nazi groups were largely unsuccessful.
Rebel forces have been infiltrating into the country.
to secretly put people into an organization or place in order to find out information or to harm it
infiltrate somebody into something
They repeatedly tried to infiltrate assassins into the palace.
to become a part of something - used especially to show disapproval:
Commercialism has been infiltrating universities for the past decade.
—infiltrator noun [countable]
—infiltration noun [uncountable]