There are two (or three) kinds of flacherie: basically, infectious (viral) flacherie and noninfectious (touffee) flacherie. Both are technically a lethal diarrhea.
Viral flacherie is ultimately caused by infection with Bombyx mori infectious flacherie virus (BmIFV, Dicistroviridae), Bombyx mori densovirus (BmDNV, Parvoviridae) or Bombyx mori cypovirus 1 (BmCPV-1, Reoviridae). This either alone or in combination with bacterial infection destroys the gut tissue. Bacterial pathogens contributing to infectious flaccherie are Serratia marcescens, and species of Streptococcus and Staphylococcus in the form known as thatte roga.
In 1865, Louis Pasteur was the first one able to recognize that mortality due to viral flacherie was caused by infection. He developed a method by which silkworm farmers could distinguish and discard infected eggs using a light microscope.