infiltrate

Short definition from Wiktionary

Verb

1. 1
   To surreptitiously penetrate, enter or gain access to.
2. 2
   To cause to penetrate in this way.
3. 3
   To pass through something by filtration.
4. 4
   To cause (a liquid) to pass through something by filtration.
5. 5
   To invade or penetrate a tissue or organ.
6. 6
   To send (soldiers, spies, etc.) through gaps in the enemy line.
7. 7
   To move from a vein, remaining in the body.

Noun

1. 1
   Any undesirable substance or group of cells that has made its way into part of the body.