You need to have an understanding of that area of China to know why it's so prone to disease. China is one of the most densely populated countries in the world, with 329 people per square mile. That compares to 72 people per square mile in the U.S.
The living conditions in rural China are still primative. Sanitation is a concept that's yet to be followed in daily practice. Most places in China still use a squat toilet.
That region is also in the tropical zone, so it's prone to the worst kinds of parasitic mosquito-born diseases known to man.
Considering that all three of these elements are present in that one small area of southern China, it's not surprising in the least to see so much disease there.