Infiltrator (noun)

1. A person who enters a group, organization or place stealthily and secretly, often with the intent of obtaining information or causing disruption.
2. A spy or a saboteur who enters an organization or group from within.

Origin:

This word is formed by the combination of the prefix "in-" meaning "into" or "within" and the word "filtrator" meaning "one who filters" or "one who enters stealthily".

Examples:

1. The spy was discovered to be an infiltrator who had infiltrated the organization for months.
2. The group was infiltrated by an infiltrator who was working for the government.
3. The infiltrator was able to gather valuable information about the organization's plans.
4. The infiltrator caused significant damage to the group before he was discovered.
5. The infiltrator was able to blend in with the group and gain their trust.