There were many great answers on the previous chapter and today I plan on presenting an additional one. As you can already tell based on the chapter title, today is about malaria. Malaria is a disease that has been extremely successful at killing humans throughout history. Thankfully the numbers have however dropped significantly in recent times.
Malaria is disease that's caused by a parasite that's mainly spread by mosquitoes. Thus parasites and mosquitoes would directly and indirectly also be on the top of the leaderboards for kills.
The number of deaths caused are ludicrous to the degree that many people believe malaria has killed half of all humans to ever exist. Even still in 2020 there were an estimated 241 million malaria cases and 627,000 deaths. Even back in the 20th century, malaria was blasting hundreds of millions of kills.