Do Mosquitoes Really Cause Malaria? The Case for Béchamp's Terrain Theory

The mosquito is called the deadliest animal on Earth. The World Health Organization estimates malaria caused 282 million cases and 610,000 deaths in 2024, and the mosquito gets the blame for every one of them. Ask anyone how malaria works and they will tell you with total confidence: a mosquito bites you, you get malaria.

Now ask them how they know. Nobody has watched a parasite jump into their arm. Most people bitten by mosquitoes never get malaria. For two thousand years the best doctors alive were certain the disease came from the swamp itself, and they had the evidence of their own eyes. One of Louis Pasteur's great rivals, Antoine Béchamp, spent his life arguing that germs follow disease rather than cause it. Pasteur himself is quoted as conceding on his deathbed that the rival was right.

This is a debate piece. It gives the underdog the floor: the case that malaria is about the terrain, the body and the environment it lives in, and that the mosquito is history's most convenient scapegoat. Mainstream science gets its say near the end. You decide.

One rule before we start: this is a debate, not medical advice. If you live in or travel to a malaria area, use nets, repellent and the antimalarials your doctor recommends while you make up your mind.

Most people have never asked how they know a mosquito causes malaria. That is exactly why it is worth asking.

Before the argument, some history that rarely makes it into the clean version of the story. Every one of these is documented.

• 1. Pasteur kept secret notebooks

  Pasteur asked his family never to show his private lab notebooks. When the historian Gerald Geison finally studied them, a century later, they showed that Pasteur's famous 1881 anthrax vaccine demonstration used a method closer to a rival's than the one he described in public.

• 2. The "germ" was first called debris

  When Alphonse Laveran described malaria parasites in the blood in 1880, many of the leading experts of the day dismissed what he saw as fragments of broken blood cells.

• 3. A Nobel Prize for giving people malaria

  In 1927 the Nobel Prize in medicine went to Julius Wagner-Jauregg, who deliberately gave patients malaria. The high fevers could halt late-stage syphilis of the brain. Fever, in other words, was the medicine.

• 4. The body fights back in its DNA

  The sickle-cell gene, and the missing Duffy antigen in most people of West and Central African ancestry, spread through whole populations because they protect against malaria. The human terrain itself evolved around this disease.

• 5. The terrain man made the first germ-killer

  Béchamp synthesised arsanilic acid in 1859. That arsenic chemistry led Paul Ehrlich to Salvarsan in 1910, the first modern targeted antimicrobial drug. The father of terrain theory helped invent the magic bullet.

The word comes from the Italian mal'aria, "bad air". For centuries the fevers of the Roman Campagna, the English Fens and the American South were blamed on miasma: foul vapours rising from marshes, especially at night. In English the disease was simply called ague or marsh fever.

This was not superstition. It was a theory built on careful observation. Fevers clustered near standing water. They peaked in warm, wet seasons and after dusk. And the cure worked: drain the marshes, and the fevers retreated, decades before anyone had heard of a malaria parasite.

Terrain advocates point out that the oldest name for this disease describes an environment, not an insect. The question they ask is simple: what if the old doctors were closer to the truth than we give them credit for?

Louis Pasteur (1822–1895) and Antoine Béchamp (1816–1908) were both serious French scientists, both trained in chemistry, and they spent decades accusing each other of bad science and stolen ideas.

Pasteur became the public face of germ theory: specific microbes, entering from outside, cause specific diseases. His fermentation work, his experiments against spontaneous generation, his anthrax and rabies vaccines and the technique still called pasteurisation made him a national hero.

Béchamp championed what is now called terrain theory. In his view the body's internal state, its "terrain", decided health and disease. He described tiny particles inside every living cell that he named microzymas ("little ferments"), which he said survived the death of the organism and could transform into bacteria when the terrain deteriorated. Microbes, on this reading, were a result of disease rather than its cause, and they could change form depending on conditions, an idea called pleomorphism. Pasteur held that microbial species keep a fixed identity, sometimes called monomorphism.

It started with silkworms

Their rivalry has an animal origin. In the 1860s a disease called pébrine was wrecking France's silk industry, killing domestic silkworms in their millions. Both men studied it, and both claimed credit. Pasteur also identified a second silkworm disease, flacherie, and noticed something telling: heat, humidity and poor feeding made worms far more vulnerable. Even the champion of germs found himself studying the terrain.

Why did Pasteur win?

Béchamp's supporters argue that Pasteur won the argument through politics as much as proof. He was a brilliant self-promoter with powerful patrons, close to the imperial court of Napoleon III, and his version of disease was easy to sell: an external enemy you can see under a microscope, kill with a product and vaccinate against. Béchamp's version, that health depends on the whole condition of the body, offered no enemy and nothing to sell.

Béchamp, meanwhile, died in 1908 largely forgotten. His defenders call it one of the great injustices in the history of science.

The deathbed quote

The rallying cry of terrain theory is a line attributed to Pasteur at the end of his life: "Bernard was right. The microbe is nothing; the terrain is everything." Claude Bernard was the physiologist who described the body's milieu intérieur, its internal environment. Historians have not found a record of the line from Pasteur's lifetime, and it appears in later terrain literature. Whether or not he said it, the sentence has outlived almost everything else said in that debate.

"The microbe is nothing; the terrain is everything."

Stripped down, the two schools disagree about what causes disease, what microbes are doing and how to keep people well.

Here is the case Béchamp's modern followers make about malaria, in their terms. It is stronger than most people have been told.

1. Two thousand years of doctors can't all have been wrong

Every culture that lived with malaria tied it to place: marshes, low ground, still water, hot nights. Wealthy Romans retreated to the hills in the fever season. The English Fens were notorious for ague. Draining wetlands drove the fevers out long before the parasite was described. Terrain advocates say the environment was always the real story, and the mosquito was a late addition that let medicine stop talking about where and how people lived.

2. Most bitten people never get sick

In parts of Africa a person can receive hundreds of infective mosquito bites in a year. Many adults there carry malaria parasites in their blood and feel completely well. If the parasite is the cause, the terrain camp asks, why are so many carriers healthy? Their answer: what decides who falls ill is the strength of the body, not the bite.

3. Malaria follows poverty, not mosquitoes

Malaria hits the malnourished, the anaemic, the poorly housed and the very young hardest. Countries that became wealthier, better fed and better housed watched malaria disappear, and many of them still have plenty of Anopheles mosquitoes today. Terrain advocates say that is the cure history actually shows: fix the terrain and the disease goes, mosquito or no mosquito.

4. The fever is the body's work

Malaria's fevers come in tight cycles, every two or three days. Terrain advocates read those cycles as the body's own rhythm of cleansing and repair, not as damage done by an invader. They point to Wagner-Jauregg's Nobel Prize as proof that fever can heal: doctors once used malaria's fevers deliberately, as therapy.

5. Mosquitoes go where sickness already is

Research has found that mosquitoes are more attracted to people already carrying malaria. The terrain reading: mosquitoes are drawn to bodies that are already unwell, the way flies find a wound. They arrive with the disease. They do not bring it.

6. Follow the money

A disease caused by an insect sells insecticides, bed nets, drugs and vaccines. A disease caused by poverty, drainage and diet asks harder and more expensive questions about how people live. Terrain advocates argue that blaming a mosquito is politically convenient, and that it has been since Pasteur's day.

Whatever you conclude about malaria, the mosquito has the worst reputation in the animal kingdom, and it is worth hearing its side.

There are around 3,500 mosquito species, and the great majority never bite humans at all. Male mosquitoes never bite anything: they drink nectar, and many mosquitoes pollinate plants along the way. Mosquito larvae filter-feed in ponds and puddles and are a staple food for fish, frogs, dragonfly nymphs and other aquatic animals, and the adults feed birds and bats. Remove them and food webs from Arctic tundra to tropical wetlands would feel it.

Only females of some species take blood, because they need the protein to make eggs. The malaria story is pinned on one genus, Anopheles, and on a few dozen of its roughly 500 species. For an animal that mostly just wants to lay its eggs, that is a heavy charge.

Mosquito field guide

Medical science's position is that malaria is caused by Plasmodium parasites, which only female Anopheles mosquitoes transmit in ordinary life. The parasite completes its sexual stage inside the mosquito's gut, travels to its salivary glands and is injected with a bite. Its key test was run in 1900: two doctors, Louis Sambon and George Low, spent the malaria season in the marshes near Rome behind mosquito screens and stayed well while their neighbours fell ill. Meanwhile, infected mosquitoes shipped to London gave malaria to a volunteer who had never been near a swamp.

On the terrain points, mainstream medicine agrees that the body matters: immunity built up by surviving earlier infections, genetics, nutrition, pregnancy and housing all shape how sick a person gets. Its view is that they change the severity, not the cause. That is why WHO recommends insecticide-treated nets, preventive medicines and malaria vaccines.

A century and a half after Pasteur and Béchamp stopped arguing, their debate is louder than ever. One side says malaria is a parasite delivered by an insect. The other says it is what happens to bodies living in the wrong conditions, and that the mosquito has been framed.

Which argument was hardest to dismiss? Was Béchamp robbed, or did the better theory win? Is the mosquito a killer or a scapegoat?

Team Béchamp or Team Pasteur? Make your case in the comments wherever you found this post.

Is the mosquito a killer, or history's most convenient scapegoat?