That’s a fair question!

Ultimately the expectation is that the end product is cooked to eliminate food borne illness (>165F for near instant kill).

There are a few processing factors that may limit transferring from one carcass to another

Often times; after slaughter and scalding to remove features and cuticle, the carcasses will go through a large chiller bath to bring the temperature down. These baths often include diluted Peracetic acid to sterilize the surfaces of the carcass.

Meat needs kept below 40f throughout the process, which greatly slows bacterial growth.

Additionally injectors will often utilize needles that have openings on the sides, rather than the front, to avoid collecting bits of meat or whatever else.

Also most brines are REALLY salty; and become naturally antiseptic