To stain (in, with, blood, slaughter, etc.).
To invade or penetrate a tissue or organ.
To cause to penetrate in this way.
To surreptitiously penetrate, enter or gain access to.
To send (soldiers, spies, etc.) through gaps in the enemy line.
To pass through something by filtration.
To move from a vein, remaining in the body.
To cause (a liquid) to pass through something by filtration.
Any undesirable substance or group of cells that has made its way into part of the body.