Infestation refers to the state of being invaded or overrun by pests or parasites. It can also refer to the actual organisms living on or within a host.
In general, the term "infestation" refers to parasitic diseases caused by animals such as arthropods (i.e. mites and ticks), lice, and worms but excluding those caused by protozoa, fungi, and bacteria.
External and internal 
Infestations can be classified as either external or internal with regards to the parasites' location in relation to the host.
An external ectoparasitic infestation is a condition in which organisms live primarily on the surface of the host (though Porocephaliasis can penetrate viscerally) and includes those involving mites, ticks, and head lice.
An internal endoparasitic infestation is a condition in which organisms live within the host and includes those involving worms (though swimmer's itch stays near the surface).
Medically, the term infestation is often reserved only for external ectoparasitic infestations while the term infection refers to internal endoparasitic conditions.