Black tea is British people's favorite drink. It became popular in the late 18 century when the tax of tea was reduced. The mortality rate was reduced in this period, even though the living standard was not improved. The mortality rates before and after black tea became popular were compared between areas with bad water and good water. The rate was reduced more in the former than the latter area. Moreover water-born disease such as dysentery was reduced in the same period, while tuberculosis was not. It was concluded that boiling of water killed bacteria.
I wondered if green tea had the similar effect on Japanese people. Green tea was first imported from China in Nara period (8th century). But even in Edo period (17-19th century) tea was only for upper class people. It became popular among common people in the middle of 20th century when, I think, people already knew how to boil water to sterilize it.