“Here, nobody disputes that thousands of different compounds fall under the heading of PFAS; one of the defendant’s experts puts the number of different PFAS at 5,000-10,000, which is roughly the number of known species of mammals on Earth,” U.S. Court of Appeals Judge Raymond M. Kethledge for the Sixth Circuit said. “To allege simply that these defendants manufactured or otherwise distributed ‘PFAS,’ therefore, is patently insufficient to support a plausible inference that any of them bear responsibility for the particular PFAS in Hardwick’s blood.”