Robert Koch announces the discovery of Mycobacterium tuberculosis, the bacterium responsible for tuberculosis.

Robert Koch’s discovery of Mycobacterium tuberculosis in 1882 marked a turning point in the understanding of infectious disease. At the time, tuberculosis—often called “consumption”—was responsible for millions of deaths worldwide, yet its cause remained uncertain. Koch, building on advances in microscopy and staining techniques, set out to identify the specific organism responsible. His work was driven by a broader scientific movement seeking to connect particular diseases with specific microorganisms.

Koch employed innovative laboratory methods to isolate and visualize the bacterium. Using a special staining technique, he was able to make the otherwise difficult-to-see organism visible under a microscope. He then successfully cultured the bacterium outside the human body, which was a major technical achievement at the time. By infecting experimental animals with the cultured bacteria and observing the development of disease, Koch demonstrated a direct causal relationship between the microorganism and tuberculosis.

This work led Koch to formalize a framework for linking microbes to diseases, now known as Koch’s postulates. These principles established that a microorganism must be found in all cases of a disease, isolated and grown in pure culture, and capable of reproducing the disease when introduced into a healthy host. His discovery of *Mycobacterium tuberculosis* became one of the most compelling validations of this framework and helped solidify the emerging field of bacteriology.

The impact of Koch’s discovery was profound and long-lasting. It transformed tuberculosis from a mysterious and often hereditary-suspected condition into a clearly defined infectious disease, paving the way for improved diagnostics, public health measures, and eventually treatments. Koch’s announcement on March 24, 1882—now commemorated as World Tuberculosis Day—remains one of the most significant milestones in medical history, laying the foundation for modern microbiology and infectious disease research.