Howie Mandel -- Intruder CONTAMINATED My Home!

Howie Mandel is experiencing every germaphobe's WORST nightmare -- discovering an intruder was lurking in his home ... and possibly TOUCHED SOMETHING!

Law enforcements sources tell TMZ -- Howie was out of town when a maid discovered an open door in his L.A. area home.

We're told it's unclear if anything was stolen -- but Howie's people filed a police report anyway.

Howie is now conducting an extensive inventory of the home -- to make sure nothing is missing.