There is actually a disease listed as 'delusional parasitosis' and they will often use that as their diagnosis as parasite infestation is not found in their database of illnesses. It is cheaper, easier, and more profitable for them to use that standard diagnosis than it is to buck the system.
Also, if diagnosed as having 'delusional parasitosis' which is a mental condition, that can be used to take things away such as your second amendment rights because you may not be stable if you are so delusional as to having seen parasites or have strange itching, rashes, etc.