Infection and infestation are two terms closely related to each other and very frequently used in various branches of health sciences and life sciences in general. Both refer to processes of invasion of one organism by another and are often used synonymously. If you consult the definition of infect and infest in the RAE dictionary, you can see how easy it is to fall into confusion, because not even medical dictionaries and specialized portals define both terms unequivocally and homogeneously.
From a medical point of view, both infection and infestation refer to the action and effect of an organism that invades another, developing in or on it. Both are further defined as pathological processes and, therefore, cause damage to the host.
Where are the differences?
The difference is basically in the type of invading organism. The term infection refers to invasion by microscopic organisms, eg bacteria, viruses and protozoa, while infestation refers to invasion by macroscopic organisms, eg intestinal helminths. In this sense, the term infestation is much more used in the context of parasitology and infection in the context of microbiology.
This differentiation between infection and infestation is usually quite common but it is not consistent between different sources consulted. For example, the Clínica Universidad de Navarra defines the term infestation as invasion by any macroscopic parasite while the medical dictionary of the University of Salamanca limits it only macroscopic ectoparasites (they live outside the host, on it). Both definitions differentiate the type of invading organism but also the location of the invasion. Other sources, such as the MeSH, define infection as invasion by microorganisms that can have pathological consequences but do not define infestation, and when defining parasitic disease, define it as infection or infestation by parasitic organisms without giving further details.
Let's take an example: the Renal dioctophimosis is the invasion of the kidney by Dioctophyma renale, a macroscopic nematode known as a giant kidney worm (females can reach 60–100 cm in length and 7 mm in width). This disease could be an infestation, since it is produced by a macroscopic organism, but it could be an infection because it is an internal invasion. In fact, you can consult documents about this disease that talk about infection and others that talk about infestation.
So there is no difference?
Well, it seems that the only clear difference is that infection is preferred to talk about invasion by microorganisms (bacteria, protozoa, viruses and fungi) and that infestation is preferred for macroscopic parasitic organisms. It is in cases of internal invasion by macroscopic organisms that most of the confusion seems to lie, and in these cases both terms could be used synonymously.
Don Daniels, in a reflection on the Bionet parasitology channel8investigates the etymological origin of the terms and reaches this conclusion:
- infestation: is the invasion process (action)
- infection: is the effect of the invasion (state or condition)
Infection comes from the Latin inficere, which means "stain" or "corrupt". Use with pathological significance began in the middle3. Infestation, meanwhile, comes from the Latin I will infest, which means "attack". In medieval Latin and Romance languages, both are used synonymously, beginning to associate infestation with macroscopic parasites in the 19th century.4. Based on this etymological origin, it could be said that infesting would be the action, the attack, and the infection would be the condition caused by this attack; in this sense, infestation would be the previous step for an infection.
Any doctor in the room who wants to give an opinion?