The Spanish Flu also influenced the design of old radiator heating systems.

"The Spanish influenza epidemic, which took place during the winter of 1918–19, had a dramatic effect on the way heating engineers sized radiators," explains Dan Holohan, author of several books on steam heat and founder of HeatingHelp.com. "Central heating was relatively new then and the world had never seen anything as horrible as the Spanish flu, which caused an estimated 50 million deaths. Because the flu was airborne, the Board of Health started a campaign the following year that urged (more like demanded) people to keep their windows open so fresh air could enter the rooms."[1]

[1] https://www.citylab.com/life/2014/02/our-indispensible-guide...