No, actually.
Polio isn't a bacteria, for one, and it's spread through fecal-oral transmission. You would think that sanitation would make this better and technically it does, but avoiding shit particles is pretty much impossible even in the modern era. They just get places. Recall the polio epidemic of the early-mid 20th century. Would you argue that the Ymaryn have better sanitation and medical practices?
The thing is, infants are generally immune to polio when breastfeeding, so long as their mothers were exposed to the virus at some point in their past (and thus developed immunity). In lower sanitation conditions, infants would often be exposed to the virus while they had this immunity from their mothers leading them to develop their own natural immunity. In cleaner conditions, the infant never develops their own immunity to the virus, and then loses the protection they had from their mothers milk. As a result, they are far more likely to be exposed to the virus and develop the disease without the presence of their mothers protection to blunt the effects.
It's a bit paradoxical, but cleaner living conditions actually lead to a higher incidence of polio. So, unless you can get the world completely sterile, you're gonna have a polio problem as things get cleaner.