Indeed, incidence among the inhabitants of the south China coast and their overseas descendants is widespread, and the disease sometimes strikes several members of the same family.
因為中國南方沿海居民罹患率最高，而這些地區移居他處的華僑也維持同樣的高發生率；雖然第二代、第三代華僑，發生率降低了，但仍比當地居民高。此外，還有同一家族出現多位病患的現象，在在顯示與遺傳因素有關。