in·fil·trate 
(ĭn-fĭltrāt′, ĭnfĭl-)
(ĭn-fĭltrāt′, ĭnfĭl-)v. in·fil·trat·ed, in·fil·trat·ing, in·fil·trates
v.tr.
1.
a. To pass (troops, for example) surreptitiously into enemy-held territory.
b. To penetrate with hostile intent: infiltrate enemy lines; terrorists that had infiltrated the country.
2. To enter or take up positions in gradually or surreptitiously, as for purposes of espionage or takeover: infiltrated key government agencies with spies.
3. To cause (a liquid, for example) to permeate a substance by passing through its interstices or pores.
4. To permeate (a porous substance) with a liquid or gas.
v.intr.
To gain entrance gradually or surreptitiously.
n.
One that infiltrates, especially an abnormal substance that accumulates gradually in cells or body tissues.
in·filtra·tive (-trə-tĭv) adj.
in·filtra·tor n.