While Rattles may be affected by more generalized parasites affecting monsterkind, there is a specific species of parasite focused on affecting rattles. This parasite dwells in the core of the rattle, to more easily drain and utilize their essence. It mimics a topping as to avoid drawing suspicion and to better hide, as the contents of the tail are clearly visible. When smaller and younger, it is relatively harmless. However, as it ages it gains the ability to exercise control over the host.
Though usually harmful to the host, instances of symbiosis have been observed. Being quite intelligent and driven to survive, on rare occasions parasites may be persuaded to do a sort of symbiosis rather than a full hostile takeover of the body and mind.
Baby parasites can survive for a limited time outside the host they were born in, long enough to slip into a new one through a crack or down a straw. Initially, they are really tiny and all look the same, but they grow and develop traits depending on the core they grow up in.
Younger parasites tend to look more like the actual topping, whereas the older and more established (and therefore harder to remove) they get the more alien they look as subtly becomes less essential.