How did the germ theory of disease develop?

In Antiquity, the Greek historian Thucydides (c. 460 – c. 400 BC) was the first person to state, in his account of the plague of Athens, that diseases could spread from an infected person to others. One theory of the spread of contagious diseases that were not spread by direct contact was that they were spread by “seeds” (Latin: semina) that were present in the air. In his poem, De rerum natura (On the Nature of Things, ca. 56 BC), the Roman poet Lucretius (ca. 99 BC – ca. 55 BC) stated that the world contained various “seeds”, some of which could sicken a person if they were inhaled or if they contaminated his food. The Roman statesman Marcus Terentius Varro (116–27 BC) wrote, in his Rerum rusticarum libri III (Three Books on Agriculture, 36 BC): “Precautions must also be taken in the neighborhood of swamps […] because there are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and there cause serious diseases.” The Greek physician Galen (129 AD – ca. 200/ca. 216 AD) speculated in his On Initial Causes (ca. 175 AD) that some patients might have “seeds of fever”. In his On the Different Types of Fever (ca. 175 AD), Galen speculated that plagues were spread by “certain seeds of plague”, which were present in the air. And in his Epidemics (ca. 176–178 AD), Galen explained that patients might relapse during recovery from a fever because some “seed of the disease” lurked in their bodies, which would cause a recurrence of the disease if the patients didn’t follow a physician’s therapeutic regimen

During the Middle Ages, Isidore of Seville (ca. 560 – 636) mentioned “plague-bearing seeds” (pestifera semina) in his On the Nature of Things (ca. 613 AD). And in 1345, Tommaso del Garbo (ca. 1305–1370) of Bologna, Italy mentioned Galen’s “seeds of plague” in his work Commentaria non parum utilia in libros Galeni (Helpful commentaries on the books of Galen).

The Italian scholar and physician Girolamo Fracastoro proposed in 1546 in his book De Contagione et Contagiosis Morbis that epidemic diseases are caused by transferable seed-like entities (seminaria morbi) that transmit infection by direct or indirect contact, or even without contact over long distances. The diseases were categorised based on how they were transmitted, and how long they could lie dormant