Infiltration

In medicine, infiltration is used to describe any higher than normal diffusion or accumulation of material in tissue or organs that is not a normal component of such tissues or organs. Such material is referred to as infiltrate. Infiltrates are usually materials produced by the body itself as part of a disease, such as cancer cells or amyloid proteins. White blood cells are another common infiltrate as they move to the site of an infection. They are a common complication of intravenous lines.
Infiltration at Wikipedia