Within the world of bees, wasps and ants (hymenoptera) are many species that are called parasitoids. A parasitoid is an organism that preys upon another organism in a similar fashion as a parasite. The difference is that a parasite does not which the host to die whereas a parasitiod kills the host.. in this case the hornet. The process is that the parasitoid which is usually a wasp though sometimes it can be specific species of fly lays an egg on the hornet which then hatches and burrows into the horent ( or other insect depending upon the species of parasitoid.) Once it has entered the host it can either mimic the immune system of the host and go undetected, it can encapsulate itself and go undetected, it can destroy the immune system all together, etc. The end result is that the larva of the parasitoid consumes the host from the inside out.