수능완성 실전편 36번 than 해석 질문

유사관계대명사 than의 선행사가 궁금해요. 저는 countless diseases라고 생각했는데, 해설에는 higher human densities와 연결된다고 되어 있네요.
아래 문장에서 than이 어디를 받는지 어떻게 봐야 할까요?
“Therefore, a rise in human population size or density can expose the population to a disease that previously could not be sustained in the population. The advent of agriculture, for example, resulted in the transmission of countless diseases that require higher human densities than occur in hunter-gatherer societies.”