The title of this article is actually underselling it. They specifically give them disease-infected piglets and their feces

Employees can be seen removing the intestines of dead, disease-infected piglets and mixing them with piglet feces in a blender — a mixture to be fed to the adult breeding pigs — causing one worker to gag.

The practice, called “feedback,” is common in the pork business (or “controlled oral exposure” in industry jargon). The slurry of pig poop and parts is often fed to new female breeding pigs

https://www.vox.com/future-perfect/23817808/pig-farm-investigation-feedback-immunity-feces-intestines