For centuries, people believed diseases like tuberculosis came from “bad air” (miasma) or divine punishment. The discovery of microbes, and the development of germ theory, revolutionized that understanding. 

In 1882, Robert Koch identified the bacteria Mycobacterium tuberculosis as the cause of TB. To prove this, he developed a set of principles known as Koch’s postulates, which laid the foundation for modern microbiology: 

1. Find it – The germ should be present in all cases of the disease, but not in healthy individuals (people who don’t have a disease). 

1. Isolate it – The germ must be isolated and grown in pure culture. 

1. Test it – Introducing it into a healthy organism should reproduce the disease. 

1. Prove it – The same germ must be recovered from the newly infected organism. 

Although modern science has revealed exceptions (some microbes can’t be cultured (that is, grown in the lab) or don’t always cause disease), Koch’s approach remains a cornerstone of scientific rigor in infectious disease research.