Germs and other microorganisms were first observed as early as the 17th century. Antonie van Leeuwenhoek, in the 1670s, invented the microscope and is credited with being the first to see and describe bacteria and other microbes. However, at that time, the prevailing belief was the theory of spontaneous generation, which held that germs were created by decaying matter rather than causing disease. The modern germ theory—that specific germs cause specific diseases—was developed in the 19th century. Louis Pasteur published his Germ Theory in 1861, proving that microbes in the air caused decay and disease, not the other way around. Robert Koch furthered this work in the 1870s-1880s by identifying specific microbes that caused particular diseases such as anthrax, tuberculosis, and cholera. This period saw the key discoveries and acceptance of germ theory, leading to major advancements in medicine and public health. So, germs were discovered in the 17th century (1670s), but the discovery that germs cause disease—the germ theory—was established and proven mainly in the 19th century, with Pasteur's work in 1861 and Koch's contributions in the 1870s-1880s.