In Lyon, silk production began to be industrialized, reaching a peak fueled by society's growing interest in silk fabrics at the time. So, when mid-19th century silkworm farmers observed the mass death of their tiny workers, the economic impact of the problem led them to seek solutions from all areas, including science.

It was 1865, and the plague that was killing silkworms in France was not only a threat to that country's economy but also to other producers such as Italy, Austria, and Asia Minor.

Some experts had made progress in explaining the phenomenon, but it was Pasteur who finally solved the mystery. In his laboratory, the scientist focused on studying the worms and discovered two diseases affecting them: pebrine and flacherie .

Through experiments and countless hours observing the worms under the microscope, Pasteur described pebrine as a contagious and hereditary disease caused by a fungus, which led to small dark spots on the caterpillars and was fatal within a few days. Flacherie, on the other hand, was caused by a bacterium that affected the worms' digestive system, making them sluggish and lethargic before ultimately killing them.

But perhaps more important than identifying the cause of the problem, Pasteur's contribution lay in understanding how the disease developed and why it spread among the worms, much to the dismay of producers. By identifying the cause, Pasteur was able to suggest measures to prevent its spread, such as identifying infected worms early and separating them from the rest to prevent contagion.

In the 16th century, Lyon was considered the European capital of silk.

He also introduced the idea that certain conditions favored contagion and referred to some hygiene rules, such as good ventilation and quarantining worms suspected of being sick.

At the time, these simple —but still little-known— concepts saved the silk industry from collapse. More importantly, they had an impact beyond the original problem by paving the way for the study of contagious diseases. For the first time, the issues of inheritance and contagion were scientifically proven, and hygiene standards were established.

It was now time for Pasteur to turn his focus to human diseases. 

Microbial Theory of Disease

For centuries, humanity believed that a person's health depended on the balance of four bodily fluids known as humors: blood, phlegm, black bile, and yellow bile. When the balance between these elements was disrupted for some reason, diseases would arise. Under this concept, for example, a common practice to cure certain illnesses was to induce bleeding to restore the balance of fluids.

This Theory of the Four Humors persisted for over 2,000 years. Although it had some detractors over time, it wasn't until 1858 that German physician Rudolf Virchow (1821–1902) made significant strides in human biology. Among many other concepts that propelled modern medicine forward, he proposed the idea that the cell is the origin of living organisms and described human diseases as disorders affecting the body's cells.

In this context, Pasteur and his research on microorganisms entered the scene. After studying their role in wine, beer, and silkworms, he introduced another revolutionary concept: the idea that microorganisms—invisible and still largely unknown beings—could be the cause of diseases in humans and animals.

Pasteur postulated that these microorganisms infected a host, reproduced, and caused various disorders. This led to the concepts of the "germ" —a microorganism capable of causing illness—and "infectious disease," which is caused by a germ. Based on this, Pasteur refuted the theory of humors and, in 1878, described the Germ Theory of Infectious Diseases.

Initially, this theory sparked some controversy, primarily because Pasteur was not a physician but a chemist. However, a few years later, German physician Robert Koch (1843–1910) confirmed Pasteur's theory through his research on tuberculosis, proving that it was caused by a specific microorganism. Koch received the Nobel Prize in Medicine in 1905 for this work.

Before Pasteur's ideas were widely accepted, however, his suggestion that microbes in the air could cause diseases and that illnesses could be transmitted from person to person was difficult for many to grasp. This was largely because the modes of disease transmission were still poorly understood.

As science advanced in this area, Pasteur emphasized that certain simple measures, such as isolating the sick, washing hands, and avoiding exposure to microbes, were essential to preventing disease. In 1871, despite not being a physician but confident in his knowledge of germs and pasteurization, he suggested to military surgeons during the Franco-Prussian War that surgical instruments and bandages used on wounded soldiers should be boiled to eliminate potential germs. He even described a device called the "Pasteur oven," which was designed to sterilize surgical tools more effectively—a principle still used today for sterilization.

In that vein, the contributions of Koch and British surgeon Joseph Lister (1827–1912) were also crucial in reducing mortality from infections in wounded patients and those undergoing surgery. Building on Pasteur's work on germs, Lister also linked microorganisms as the cause of wound infections and searched for a chemical substance that could kill them. He identified carbolic acid (also known as phenol), which is regarded as the first antiseptic.

Thus, from wine to silkworms, Pasteur's ideas and discoveries made a significant impact in the medical field. And that was only the beginning... there was still more to come.