Mosquitoes are very good at transmitting disease, whether it’s the Zika virus dominating current global health news or the age-old malaria that has killed billions of people over the course of human history.

It wasn’t known that mosquitoes could be dangerous, however, until the late 1870s, when a Scotsman practicing medicine in the Far East discovered that these insects can host parasites that cause human illness. His name was Patrick Manson.

Born near Aberdeen, Scotland in 1844, as a teenager Manson was apprenticed to an ironsmith, but he didn’t have a sturdy enough build to accommodate rigorous manual labor. Instead, he began medical school at the University of Aberdeen. After graduation, he worked at a mental asylum before heading across the world in 1866 to work as a port surgeon for the Imperial Chinese Customs Service in Formosa (present-day Taiwan). He was later transferred to Amoy on China’s southeast coast, where he operated on tumors and encountered a condition that fascinated him: elephantiasis.

At the time, the disease—which can be incapacitating and severely disfiguring, swelling soft tissues to colossal proportions and thickening skin—was leading people to suicide. Aside from having their social lives ruined, many were rendered unable to work. Their families often suspected them of demonic possession, owing to their drastic, horrifying change in appearance.

In 1875 Manson went to London, where he married Henrietta Isabella Thurburn, the 18-year-old daughter of a Royal Navy captain, before bringing his new bride back to Amoy with him the following year. And during his year in London, Manson did more than get married. He also frequented the British Museum’s reading room, where he researched the elephantiasis condition that was plaguing so many people on the other side of the planet.

After returning to his post in South China, Manson investigated the life-cycle of the filarial worm that was just then being established as the cause of elephantiasis. In 1877, he conducted experiments on his gardener, who was infected with the worm. The doctor had mosquitoes feed on the man while he slept, and then dissected the insects after they’d engorged themselves on the gardener’s blood.

Patrick Manson experimenting with filaria sanguinis-hominis on a human subject in China. Image credit: Wikimedia // CC BY 4.0

 
Observing the mosquitoes’ stomach contents under the microscope, Manson saw that the filarial parasites developed further in their life cycle than they did inside a human. Over the course of several days, the parasites beneath his microscope had transformed from “structureless filaria embryos into morphologically distinct larvae,” writes Douglas M. Haynes in his book Imperial Medicine: Patrick Manson and the Conquest of Tropical Disease.

Based on these observations, Manson arrived at the realization that mosquitoes serve as an incubator for parasites and an intermediary mechanism for passing them on to humans (although he didn't understand exactly how the parasite was passed—he thought that the mosquitoes transferred the parasite to water that was then drunk by humans).

He published his findings in medical journals both in China and in the UK. The medical communities in both places needed a wakeup call about mosquitoes—a main reason why much of Asia was sometimes called, at the time, the “white man’s grave.” Of course, people from all backgrounds were and are at serious risk from mosquito-borne illnesses (malaria alone killed an estimated 438,000 people in 2015, according to the World Health Organization), since the insects are very adept at introducing themselves to the human body, often unnoticed. They’re also nifty at traveling (some species can cover multiple miles), making them particularly effective—both medically and geographically—at spreading diseases.

In 1883, Manson moved to Hong Kong, where he founded the Hong Kong College of Medicine. By the end of the 1880s, having spent most of the previous 23 years in the Far East, he returned to the UK. At that point, other doctors and scientists in the Far East were continuing his work on mosquito-borne diseases.

Manson’s breakthrough with filarial parasites—showing that mosquitoes could be a disease vector for humans—formed the basis of modern tropical medicine and paved the way for the theory that mosquitoes transmit malaria. The mosquito-malaria theory would be proven in 1898 by Ronald Ross, who had been mentored by Manson, and who wrote to him: “What a beautiful discovery this is. I can venture to praise it because it belongs to you, not to me.”

The relationship between the two men would eventually become difficult. When Ross won the Nobel Prize in Physiology or Medicine in 1902, the presentation speech and Ross’s own Nobel lecture praised Manson’s influence. But soon after, Manson and Ross’s relationship soured as Ross felt that Manson wasn’t supportive enough in Ross’s disputes with other researchers. The many letters between these two ambitious, brilliant men are anthologized in The beast in the mosquito: the correspondence of Ronald Ross and Patrick Manson.

Though Manson was a Nobel nominee multiple times, he never won the prize. He was, however, knighted in 1903, if that provided any consolation. He continued his work, lecturing on tropical diseases and serving as Chief Medical Officer to the British Colonial Office. He also established the London School of Tropical Medicine, which lives on today as one of the world’s leading institutions for the study of infectious diseases.

In the opening years of the 20th century, Manson’s health began to decline as he was beset by a mix of gout and arthritis. He retired in 1912, at age 68, a self-described “permanent cripple,” whose “next attack of gout will floor [him] altogether.”

He collected honorary memberships from medical societies scattered across the globe until his death in London in 1922 at age 77. Even today, he is referred to as “The Father of Tropical Medicine.”