A lot of them had made a nest inside of the giant yōkai Gakusanjin. When Naraku stole the Fuyōheki from Gakusanjin, the parasites lost their nest and started plaguing the entire region. One of them was able to possess a Fox Deity and had it terrorize a village. Another later possessed Shippō. Inuyasha and his friends later came across a pond that was full of dead parasites. Miroku sucked them up with his Kazaana, and the other two has been knocked out by Kikyō's bow.
Sango knows very well the Parasite Chrysalis, and seeing the multitude of corpses in a pond, she understand that they have taken possession of a gigantic yōkai.
- InuYasha anime; Episode 164