Who first demonstrated that a microorganism could cause disease in silkworms?

The first person to demonstrate that a microorganism could cause disease in silkworms was Agostino Bassi. In the early nineteenth century, Bassi investigated muscardine, a devastating disease of domesticated silkworms. He concluded that the illness was caused by a living parasitic fungus, now known as Beauveria bassiana, rather than by spontaneous generation or unfavorable conditions alone.

Bassi’s work began around 1807 and continued for about 25 years. In 1835 he published his findings in Del mal del segno, calcinaccio o moscardino. Infected caterpillars became covered in a fine white powder, which was later understood to consist of millions of infectious fungal spores. Bassi showed that the disease could spread between silkworms and recommended hygiene, isolation and destruction of infected animals.

His experiments were an important early step toward germ theory, decades before Robert Koch established rigorous links between particular bacteria and diseases. Bassi later proposed that microorganisms might also cause diseases in plants, animals and humans. He did not discover a bacterium: the silkworm pathogen was a fungus, but it was still a microorganism capable of causing disease.