Two types of infections i.e. from mother moth through eggs, called transovarial contamination & through ingestion of contaminated leaf called as transoral contamination.
The disease is transmitted from the infected mother to the offspring by transovarial/ transovum means and this is called primary infection. If infection is primary, more than 50% larvae die before 3rd moult and rarely any larva go for spinning.
Although transmission through egg is the principal sources of communication of the disease, contamination of leaf by pebrine spore can take place in natural state. When healthy larvae get infected through contamination during rearing, it is called secondary infection. Secondary infection during early 4th larval stages leads to formation of flimsy cocoons. Whereas larvae infected during 5th larval stage form well formed cocoons.