Infect vs. infest

There are two main differences between infect and infest. First, infection involves germs or viruses, while infestation involves a menacingly large number of pests or parasites (e.g., mosquitos or rats). Second, an infection applies to a body, while an infestation applies to a place. For example, when rats infest a city, they might carry diseases that can infect people.

These words get tricky when used metaphorically, but it’s easy to keep them straight if you remember the literal distinctions. For example, corruption can infect a government if we think of the government as a metaphorical body, and bad thoughts can infest the mind if we think of the mind as a sort of place and the bad thoughts as pests.



It’s amazing how many of the academy’s bad ideas leak out from campus and begin to infect the body politic. [National Review]

The infamous computer virus Stuxnet was able to infect computers that have no external connection. [Wall Street Journal]

Rinderpest can infect cattle, buffalo and yaks, as well as swine, giraffes and kudus. [Los Angeles Times]


As a seasoned professional farmer, he knows the field needs to be reploughed before pests infest the weedy growths left behind. [Telegraph]

Rural NSW is has been infested with the worst mouse plague in 50 years. [The Canberra Times]

2 thoughts on “Infect vs. infest”

  1. The place-vs.-body concept is subtle, but apt.

    Inanimate things can not be infected, only infested.
    And your body can suffer an infestation, e.g. of lice or worms, but in such cases your body is acting as the place of the infestation.

  2. How about the specific case of a parasite, such as those found in the “infested” waters of the Nile? If you bathe in the Nile, you can become infested with parasites, because the source is an infestation.

    My question is not about a place, but about ones body that can be infested/infected with one or more blood-borne parasites that cause a disease or condition when the source is another person.

    Does the body truly act as the “place” of the infestation if it occurs in/during blood to blood contact in the same manner as one would infect a sexual partner with HIV?

    Does the physical existence/definition of a parasite require the use of the verb infest? Can a parasite ever infect a person, or does it have to infest it?

    Would you need to use both verbs to describe a case where one is infected by a virus and simultaneously infested by a parasite?

    Can an infestation of a parasite cause an infection? If so, how do you refer to the parasite? When you then refer to the disease, do you have to state “He was infected with X due to an infestation of Y parasite.”?

    I apologize for the convoluted mess. I’m trying to get my references correct in my writing.


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