An infection is defined as the invasion and multiplication of an infectious agent in or on a host; this infection may or may not cause disease.

An infestation is the invasion, but not multiplication, of an organism in or on a host. Because many metazoan parasites do not multiply within the same host, the presence of these agents is most appropriately referred to as an infestation.

However, these two terms are also commonly used to contrast the presence of organisms inside a host body (infection) with the presence of organisms on the hair, fur, feathers, or skin of a host (infestation).

B. Burgdorferi isolated from the midgut of a deer tick; taken with an electron microscope.

Ixodes scapularis. Left to right nymph, adult male and adult female.

Describing the transmission dynamics of Lyme disease provides an opportunity to use both terms; the tick vector Ixodes scapularis infests a host while the bacterial pathogen Borrelia burgdorferi is transmitted by the tick and infects the host.