Usage Note
Infiltración is feminine: la infiltración. It covers both the literal medical sense — fluid or cells accumulating in tissue (infiltración de líquido en la rodilla) — and the figurative sense of covert penetration of an organisation by an agent. Medical and journalistic registers both make frequent use of the word. The related verb is infiltrar / infiltrarse.
Examples
"El médico diagnosticó una infiltración en la rodilla."
Natural Translation
The doctor diagnosed an infiltration in the knee.

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