Biological Isolation Garment

During the build up to the manned lunar landing, there was growing concern that the astronauts might bring back some sort of "moon germs" and infect the Earth; no Earthlings would have immunity to this foreign plague, the thinking went, and there would be mass extinctions as the lunar microorganisms ran rampant. Refer to Section V of the Biomedical Results of Apollo or Chapter 7 of Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions for additional (and less dramatic :-) accounts of the proposed precautions.

What was eventually implemented was the "biological isolation garment", or BIG (plural, BIGs): This was a coverall-type garment with a breathing apparatus which would prevent those pesky lunar organisms from ravaging the Earth. Refer to NASA pictures KSC-69PC-467 and S69-40753 for pictures of the Apollo 11 astronauts in their BIGs.

Of course, scientists examining the returned lunar samples found the moon completely devoid of life, and subsequent crew members did not have to wear BIGs upon return to Earth; the crews of Apollos 12 and 14 did have to wear breathing apparatuses as a continued precautionary measure, but the final three crews boarded their recovery carriers unfettered.