另外，可以看看wiki如何解释中国乙肝盛行

http://en.wikipedia.org/wiki/Hepatitis_B_in_China#Transmission

Transmission

The reason for this increased HBV infection is unknown, because hepatitis B has no clear transmission routes in many people in China, although both neonatal infection and horizontal transmission during early childhood are still the most common routes. During and before the Cultural revolution many of the cases came from unsafe needles that carried the virus. This was due to the fact that medicine in that time was rather poor in quality and was reduced to one room hospitals.

大意是说，中国乙肝病人的在很多情况下传染途径不明，不过最主要的是母婴传染和婴幼儿时期的横向传染。文革前医疗条件简陋，不安全使用注射针头也导致了大量感染。