Ronald Ross found malaria parasites developing inside a mosquito on 20 August 1897 while working in Secunderabad, India.

Ross, a British doctor in the Indian Medical Service, had been investigating the proposed connection between mosquitoes and malaria. French physician Alphonse Laveran had already identified the malaria parasite in human blood, and Patrick Manson had argued that mosquitoes transmitted it. Ross sought experimental evidence.

On 16 August, mosquitoes had been allowed to feed on a malaria patient named Husein Khan. Ross dissected members of the group over the following days. By 20 August only a few remained.

That afternoon he dissected another. Examining its stomach wall under his microscope, Ross found rounded bodies containing clusters of dark pigment. The pigment helped him recognise that he was looking at a stage in the malaria parasite’s development inside the mosquito.

Ross recorded his observations in his notebook and drew the structures he saw. The page survives, dated 20 August 1897.

The experiment did not establish every stage of malaria transmission. Ross continued his investigations, including work with malaria in birds, which helped demonstrate the parasite’s development and transmission through mosquitoes.

In 1902, he received the Nobel Prize in Physiology or Medicine for his work on malaria. The anniversary of the 20 August experiment later became World Mosquito Day.

Ross’s notebook provides a direct record of the experiment, with his notes and drawings from the day.