Infiltrating

verb

1. To surreptitiously penetrate, enter or gain access to.
2. (of a liquid) To pass through something by filtration.
3. To cause (a liquid) to pass through something by filtration.
4. To invade or penetrate a tissue or organ.

   "High-grade tumors often infiltrate surrounding structures."

5. To send (soldiers, spies etc.) through gaps in the enemy line.