There are several parasites that can infect hornets and other members of the wasp family. One example is the wasp-waisted fly (Aphanius fasciatus), which lays its eggs on the body of a hornet. When the eggs hatch, the larvae burrow into the hornet's body and begin to feed on its internal organs. Eventually, the larvae emerge from the hornet's body, killing the host in the process.
Another example of a hornet parasite is the nematode Spauligodon voynevi, which infects the head of the hornet and causes it to become disoriented and unable to perform normal functions. The nematode also causes the hornet's head to become swollen, which can be easily noticed by other hornets and can lead to the infected individual being ostracized from the group.
There are also several species of mites that can infect hornets and other wasps, including Varroa destructor, which is a major pest of honey bees. These mites can weaken and eventually kill the host wasp by feeding on its blood and tissues.
It is worth noting that while parasites can have significant impacts on hornet populations, they are also an important part of the ecosystem and play a role in maintaining balance in the environment.