[MichaelCollins]

It has nothing to do with mortality rates generally, and is specifically about polio. Polio generally wasn't killing very young infants, most of them were protected by antibodies from their mothers.

My guess is that sanitary conditions during infancy were still a net positive overall. Just not with respect to polio specifically. To protect children from polio you either need vaccination with dead/weak polio, or exposure to polio as an infant when they had protection from maternal antibodies. Either way teaches the immune system to fight polio, but without either of those the risk of polio rises with age. An adult who catches polio is about 10x more likely to die than a child (not infant) who catches it.