in general, the term "infestation" refers to parasitic diseases caused by animals like arthropods (i.e. mites, ticks, and lice) and worms, but excluding (except) conditions caused by protozoa, fungi, bacteria, and viruses, which are called infections.Infestations are often classified as either external or internal with regards to the parasites' location in reference to the host. External or ectoparasitic infestation may be a condition during which organisms live totally on the surface of the host (though porocephaliasis can penetrate viscerally) and includes those involving mites, ticks, head lice and bed bugs. an indoor (or endoparasitic) infestation may be a condition during which organisms live within the host and includes those involving worms (though schistosome dermatitis stays near the surface).Medically, the term "infestation" is usually reserved just for external ectoparasitic infestations while the term infection refers to internal endoparasitic conditions