IB Biology HLDisease & ImmunityPaper 1 & 2~8 min read
Zoonoses
Most pathogens can only infect one species. A few can jump. Those few cause most of the outbreaks that make the news, and the reason they can jump comes down to something you already know: whether the pathogen fits the receptors on the new host’s cells.
📘 What you need to know
Some diseases are species specific. Others can cross species barriers and infect several different species.
A disease may be unable to cross the species barrier if the new species does not possess the necessary receptors.
It may also fail if the body temperature of the new organism does not reach the temperature required for the disease to develop.
Zoonotic diseases are those that can cross the species barrier from animal to human.
They are a growing global concern, because close contact between humans and animals makes them difficult to control and eradicate.
They may lead to pandemics, such as the one caused by COVID–19.
Animal products may also be affected by zoonotic disease, which poses a further problem.
Some zoonotic diseases emerge from animal populations and then develop into human–only strains, as HIV did.
What stops a pathogen crossing
A species barrier is not a wall. It is a mismatch. The pathogen needs to attach to and enter the cells of the new host, and if the shapes do not fit, nothing happens.
This is the same complementary shape idea you used for antibodies and receptors earlier in the topic. Here it decides whether a disease stays in one species or moves into another.
If a question asks why a disease of one animal cannot infect humans, these two reasons are almost always what the mark scheme wants.
What a zoonosis is
Definition
A zoonotic disease is one that can cross the species barrier from an animal to a human.
Zoonoses are a growing global concern. Humans live in close contact with animals — as pets, as livestock, and through wildlife — which makes these diseases difficult to control and eradicate. Even if we clear the disease from the human population, it may still be circulating in the animal population, waiting.
Two further consequences worth remembering:
They may lead to pandemics, such as the one caused by COVID–19.
Animal products may be affected too, which poses problems for food supply as well as for health.
Once it has crossed, it may never go back. Some zoonotic diseases initially emerge from animal populations and then develop into human–only strains. HIV is the standard example — it began as a simian immunodeficiency virus in chimpanzees and now spreads human to human.
Which diseases are zoonotic
Disease
Species first infected
Status
Bird flu
Geese
Zoonotic
Tuberculosis
Believed to be humans
Zoonotic — it also moves between cattle and people
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
Notice which diseases we have actually managed to eradicate or nearly eradicate: smallpox and polio, both human only. That is not a coincidence. With no animal reservoir, vaccinating enough people finishes the job. With a zoonosis, there is always somewhere else for the pathogen to hide.
🧠
Zoonotic = it came from the zoo
Silly, but it sticks. Zoo for animals, and the disease made the jump from them to us.
Worked examples
WE 1
Explain why some diseases are species specific
Explain why a pathogen that infects one species may be unable to infect another. (2 marks)
Point 1: receptors
The second species may not possess the necessary receptors on its cells, so the pathogen cannot attach to or enter them.
Point 2: temperature
The body temperature of the second species may not reach the temperature required for the disease to develop.
No matching receptors, or the wrong body temperaturetwo marks usually means both reasons — do not write one of them twice
WE 2
Explain why zoonoses are hard to eradicate
Suggest why a zoonotic disease is more difficult to eradicate than a disease that only infects humans. (3 marks)
Point 1: the reservoir
A zoonotic pathogen also survives in animal populations, so clearing it from humans does not remove it.
Point 2: re–infection
Humans live in close contact with animals, so the pathogen can cross into people again at any time.
Point 3: the comparison
A human–only disease such as smallpox or polio has no animal reservoir, so a vaccination programme covering enough people can eradicate it completely.
You cannot vaccinate your way out if the pathogen lives somewhere else toonaming smallpox or polio as the contrast makes the argument much stronger
WE 3
Apply it: a new virus in bats
A virus is found in a bat population. Suggest what would have to be true for it to cause an outbreak in humans. (3 marks)
Point 1: crossing the barrier
The virus would need to bind to receptors present on human cells, and human body temperature would need to allow the disease to develop.
Point 2: contact
There would have to be close contact between the bats and people, directly or through another animal, for the first infection to occur.
Point 3: spread
For an outbreak rather than isolated cases, the virus would then need to spread from human to human.
Right receptors, real contact, and then person to person transmissionthe third point is the one students miss — a single crossing is not yet an outbreak
💡 Exam tips
Define a zoonosis as crossing the species barrier from animal to human.
Give both reasons a disease may not cross: receptors and body temperature.
Use the phrase animal reservoir when explaining why eradication is hard.
Know at least three zoonotic examples: bird flu, bubonic plague, COVID–19.
Know at least two human–only examples: measles, polio.
Mention that animal products can be affected as well as people.
⚠ Common mistakes
Saying zoonotic means any disease animals get. It means one that can cross to humans.
Saying all diseases can cross species. Most are species specific.
Giving only the receptor reason. Body temperature is a marking point too.
Saying HIV is currently zoonotic. It began as a species jump and is now a human–only strain.
Confusing a zoonosis with a vector–borne disease. A vector carries the pathogen without necessarily being made ill by it.
Assuming a species jump automatically causes a pandemic. Human–to–human transmission has to follow.
Up next: Vaccines & Immunity. We have just said a human–only disease can be wiped out by vaccinating enough people. The next page explains exactly how a vaccine does that, and what “enough people” actually means.
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