He discovered that the muscardine disease of silkworms was caused by a living, very small, parasitic organism, a fungus that would be named eventually Beauveria bassiana in his honor.

In 1844, he stated the idea that not only animal (insect), but also human diseases are caused by other living microorganisms; for example, measles, syphilis, and the plague.

His studies of 1807 concerned mal de segno (also known as muscardine, after a French candy), a lethal disease of domestic silkworms (Bombyx mori).

He published the results of his investigations in a paper entitled Del mal del segno, calcinaccio o moscardino (1835), stating that a living entity was the culprit, and that it was contagious;[1] we now know that the powdery appearance on the killed silkworms is caused by the production of millions of infectious white fungal spores on the dead insect (see Beauveria bassiana).

[2] From this work he expanded on a theory explaining that many diseases of plants, animals and human beings were caused by pathogenic organisms.