Back-contamination
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Back-contamination is the informal but widely-employed name for the introduction of microbial extraterrestrial organisms into Earth's biosphere. It is assumed that any such contact will be disruptive or at least have consequences over which human beings will have little control. The threat of back-contamination from the Moon was the main reason for quarantine procedures adopted for the Apollo program which are now considered standard procedure[1].
Back-contamination is easily misunderstood. The likelihood that a human being or any other animal could literally acquire an alien virus is effectively nil, as viruses are host specific. This does not mean that extraterrestrial microbes cannot act upon one pathogenically: spores might use an organism's body as hosts, while the ingestion of bacteria in any form could produce toxic chemicals. When human beings ingest contaminated food, for example, they are not acquiring a virus in the manner of the flu but the experience may still be lethal because of toxic compounds.
Further, the possibility exists that a microbe might aggressively metabolize some Earth resource were it introduced here, altering atmospheric conditions or the water cycle.
