There is still starvation
I would argue that starvation and susceiptibilitysusceptibility to disease are intertwined. The people who did the most dying of diseases were the ones who were starving. It is worth noting that European population exploded in the 1700s to 1800s; before modern and universal medicine, and before widespread immunizations. However, this was after an Agricultural revolution increased the farming yield, and after improved transportation technologies allowed food to be moved from distant places to combat local famine.
So I assert that starvation is more important than disease as a means of population control. A lot of medieval disease deaths simply took people that would have starved anyways. But even with no diseases, people aren't going to make it if there isn't enough food.