Smallpox has been killing humans since 10,000 BC. In the 18th century, over 400,000 people died every year in Europe from smallpox.
Bubonic plague went through Europe and the Mediterranean from 1346 until 1353. More than 50 million people died which was 60% of Europe's entire population at the time.
But the oldest and most lethal disease still killing humans is tuberculosis. For all the hand wringing about coronavirus, the fact is TB kills about the same number of people that corona has killed so far
each year. It's been confirmed to have been around 35,000 years ago but some believe it's been around for at least 2.6 billions years ago. TB exploded in Europe in the 17th century, with 1 in five dead listed being caused by TB in London. It is believed that in the last 200 years TB has killed over a billion people.
With the exception of smallpox, these diseases are still with us, right now, in cities and the countryside. Some believe smallpox, in the form of monkeypox or other zoonotic variants, will make comeback in lethal form. Cornonavirus is, like all things from Communist China, small potatoes.
As far as wars go WWII is the big winner, like
@Solid Snek detailed.
