Most pathogens are picky. They infect one species and are harmless to everything else. Occasionally one manages to cross into humans, and when it does, nobody’s immune system has ever seen it before. That is where pandemics come from.
📘 What you need to know
Some diseases are species specific; others can cross species barriers and infect several species.
A zoonosis (plural zoonoses) is a disease that can cross the species barrier from animal to human.
A pathogen may be unable to cross if the new species lacks the necessary receptors for it to bind to and enter cells.
It may also fail if the new species’ body temperature does not reach the range the pathogen needs.
Zoonoses are a growing global concern because humans and animals live in close contact, making the diseases hard to control and eradicate.
They can lead to pandemics, such as the one caused by COVID-19.
Animal products can also be affected, which is a further problem.
Some diseases start in animal populations and then develop into human-only strains — HIV is the standard example.
What keeps a pathogen in one species
The species barrier is not a wall. It is a set of practical obstacles, and the main one is a matter of shape.
This is the same complementary-shape idea you met with antibodies and receptors. A small mutation in the pathogen’s surface protein can suddenly open up a whole new host species.
Notice how this connects to the last two pages. Mutation gives variation; a new host is a new environment; the variants that can infect it are selected. Crossing the species barrier is natural selection acting on the pathogen.
Why zoonoses worry public health experts
Humans and animals live in close contact — farming, markets, pets, wildlife trade — so opportunities to cross are frequent.
Because there is an animal reservoir, the disease cannot simply be eliminated from the human population. It can always come back.
Nobody has prior immunity to a newly crossed pathogen, so it can spread very fast and cause a pandemic. COVID-19 is the obvious recent example.
Animal products can also be affected by zoonotic disease, which adds an economic and food-safety problem on top of the medical one.
Compare that with smallpox. Smallpox infected only humans, so once vaccination broke transmission between people there was nowhere left for it to hide. That is why it could be eradicated, and why zoonoses are so much harder.
Which diseases are zoonotic?
Disease
Species first infected
Status
Bird flu
Geese
Zoonotic
Tuberculosis
Believed to be humans
Zoonotic
HIV
Chimpanzee, as simian immunodeficiency virus
Zoonotic in origin
Bubonic plague
Fleas and rats
Zoonotic
COVID-19
Unconfirmed
Zoonotic
Measles
Humans
Human only
Diphtheria
Humans
Human only
Polio
Humans
Human only
HIV is worth a second look. It emerged from an animal population as a simian virus, crossed into humans, and then developed into strains that circulate in humans alone. So a disease can be zoonotic in origin without still needing an animal reservoir today.
Definition to learn word for word
A zoonosis is a disease that can cross the species barrier from animal to human.
Worked examples
WORKED EXAMPLE
A virus infects bats but not humans. Suggest two reasons why it cannot infect human cells. [2]
Reason 1: receptorsHuman cells may not have receptors complementary to the virus’s surface proteins, so it cannot bind to them and enter.Reason 2: temperatureHuman body temperature may not reach the range the virus needs to develop and replicate.No receptor to bind to, or the wrong body temperature“complementary” is the word that turns a vague answer into a marked one
WORKED EXAMPLE
Explain why zoonotic diseases are harder to eradicate than diseases that only infect humans. [3]
The reservoirThe pathogen also lives in animal populations, so it is not removed by treating or vaccinating humans.Re-entryClose contact between humans and animals means it can cross into humans again at any time.ContrastA human-only disease such as smallpox has no reservoir, so breaking transmission between people removes it completely.You cannot vaccinate the whole animal kingdomthe contrasting example is what lifts this from two marks to three
💡 Exam tip
Define a zoonosis as crossing the species barrier from animal to human — the direction is part of the definition.
Give receptors as the first reason a pathogen is species specific, and body temperature as the second.
Have two named zoonotic examples ready and one human-only example for contrast.
Use reservoir when explaining why eradication is difficult — it is exactly the right word.
Link zoonoses to pandemics: no population has prior immunity, so spread is fast.
⚠ Common mix-up
Thinking any disease animals get is a zoonosis. It only counts if it can cross into humans.
Saying zoonotic diseases are always new. Plague and tuberculosis have been with us for centuries.
Confusing a vector-borne disease with a zoonosis. A mosquito carrying malaria is a vector, not a host that the disease crossed from.
Assuming HIV is still zoonotic. It began that way but now spreads as human strains.
Writing that the pathogen “decides” to change host. Mutation is random; a new host is simply a new selective environment.
Up next: Vaccines & Immunity — how you can gain the protection of a secondary immune response without ever having the disease.
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