Pathogens, Transmission and Body Defences (Core)
CIE IGCSE BiologyΒ· 15 min read
1. What are Pathogens?β βββββ± 3 min
Pathogen
A disease-causing micro-organism that infects living hosts and triggers disease symptoms.
Example:
Core pathogenic groups include viruses (influenza, HIV), bacteria (Salmonella, Pneumococcus), fungi (athlete's foot) and protoctists (Plasmodium, which causes malaria).
Pathogens reproduce rapidly inside their host organism, and many release harmful toxins that damage host cells and cause the visible symptoms of disease (e.g., fever, vomiting, rashes). All infectious diseases are caused by pathogens, which can spread between individuals of the same species, and sometimes between different species.
List the four main groups of pathogenic organisms, and give one example of a disease caused by each group.
- 1
- Viruses: cause influenza (flu)
- 2
- Bacteria: cause salmonella food poisoning
- 3
- Fungi: cause athlete's foot
- 4
- Protoctists: cause malaria
2. Routes of Pathogen Transmissionβ β ββββ± 4 min
Transmission
The spread of pathogens from an infected host organism to an uninfected individual.
Direct transmission: Pathogens spread via direct physical contact between hosts, including touching, sexual intercourse, or close-range cough/sneeze droplets.
Indirect transmission: Pathogens spread via an intermediate source, including contaminated food/water, touched surfaces, long-range airborne droplets, or vector organisms (animals that carry pathogens between hosts, e.g., mosquitoes for malaria).
A student catches malaria after being bitten by a mosquito carrying the Plasmodium protoctist. State if this is direct or indirect transmission, and explain your answer.
- 1
- This is indirect transmission.
- 2
- The Plasmodium pathogen is passed to the student via a mosquito vector, which is an intermediate organism between the original infected host and the student, with no direct contact between the two humans.
Exam tip:
Always link your classification of transmission type to the specific scenario given in the exam question, do not only write the generic definition.
3. First Line of Body Defencesβ β ββββ± 3 min
The first line of defences are non-specific physical and chemical barriers that stop pathogens from entering the body entirely. They work against all types of pathogens, not just one specific strain.
Skin: A tough, waterproof physical barrier that blocks pathogen entry. If cut, blood clots form a scab to seal the gap quickly.
Nose hairs and mucus: Trap airborne pathogens before they can enter the lungs.
Stomach acid: Hydrochloric acid kills any pathogens swallowed in food or drink.
Trachea and bronchi: Lining cells produce sticky mucus that traps pathogens, and cilia (tiny hair-like structures) move the mucus up to the throat to be swallowed.
Explain how the skin prevents pathogens from entering the body.
- 1
- The skin forms a tough, waterproof outer layer that acts as a physical barrier, blocking pathogens from passing through into the bloodstream.
- 2
- If the skin is broken by a cut, platelets in the blood form a clot that develops into a scab, sealing the cut to stop pathogens from entering the open wound.
4. Second Line of Body Defences & Antibioticsβ β β βββ± 5 min
If pathogens get past the first line of defence and enter the bloodstream, the second line of defence (white blood cells) activates to destroy them. There are two core white blood cell types you need to know:
Phagocytes: Engulf and digest pathogens in a process called phagocytosis. They are non-specific, so they attack any foreign pathogen they encounter.
Lymphocytes: Produce antibodies that bind to specific antigens (marker molecules) on the surface of pathogens. Antibodies clump pathogens together so phagocytes can easily digest them, and neutralise harmful toxins released by pathogens.
Antibiotic
A chemical that kills or inhibits the growth of bacteria, without damaging human body cells.
Example:
Penicillin is a common antibiotic used to treat bacterial infections such as some throat and chest infections.
A doctor prescribes antibiotics for a patient with a bacterial throat infection, but refuses to prescribe antibiotics for a patient with a viral cold. Explain these two decisions.
- 1
- Antibiotics kill or stop the growth of bacteria, so they will effectively treat the bacterial throat infection and speed up the patient's recovery.
- 2
- Antibiotics have no effect on viruses, so they cannot treat a viral cold. Unnecessary prescription would also increase the risk of antibiotic-resistant bacteria developing.
Exam tip:
You must explicitly state that antibiotics are ineffective against viruses in exam answers to get full marks for this common question.
5. Common Pitfalls
Wrong move:
Saying antibiotics can kill viruses or treat viral infections like colds
Why:
Antibiotics act only on bacteria; viruses are a different type of pathogen and are not affected by antibiotics. This is a very common exam question that students often lose marks on.
Correct move:
Only state antibiotics work on bacteria, and explicitly note they are ineffective against viruses when asked this question.
Wrong move:
Confusing phagocyte and lymphocyte function, saying lymphocytes engulf pathogens
Why:
Phagocytes carry out phagocytosis (engulfing and digesting pathogens), while lymphocytes produce antibodies. Mixing these functions results in lost marks.
Correct move:
Link each white blood cell type to its specific function clearly: phagocytes = engulf pathogens, lymphocytes = produce antibodies.
Wrong move:
Describing antibodies as killing pathogens directly
Why:
Antibodies only bind to antigens to clump pathogens together or neutralise toxins; they do not kill pathogens themselves. Phagocytes carry out the actual destruction of pathogens.
Correct move:
Explain that antibodies mark or clump pathogens for destruction by phagocytes, or neutralise harmful toxins released by pathogens.
Wrong move:
Classifying close-range cough droplet transmission as indirect
Why:
Droplet spread at less than 1 metre between people counts as direct contact, as no intermediate source is involved. Only long-range airborne droplets or droplets on surfaces count as indirect transmission.
Correct move:
Check if the scenario involves direct close contact (direct transmission) or an intermediate source/surface/vector (indirect transmission) when classifying routes.
Wrong move:
Including T-cells, B-cells or monoclonal antibodies in answers
Why:
These are Extended level or A-Level Biology content, and are not part of the Core 0610 syllabus. Including them will not get you extra marks, and may cause you to miss required core content points.
Correct move:
Only name phagocytes and lymphocytes as the two white blood cell types for Core exam answers.
6. Quick Reference Cheatsheet
Concept | Key Exam Facts |
|---|---|
Pathogen groups | Virus, bacteria, fungi, protoctist |
Direct transmission | Close contact, touching, sexual intercourse, close-range cough droplets |
Indirect transmission | Contaminated food/water, surfaces, vectors, long-range airborne droplets |
First line defences | Skin, nose hairs/mucus, stomach acid, trachea cilia/mucus |
Phagocyte function | Engulf and digest non-specific pathogens via phagocytosis |
Lymphocyte function | Produce specific antibodies to clump pathogens and neutralise toxins |
Antibiotic rules | Only kill/inhibit bacteria; completely ineffective against viruses |
7. Frequently Asked
Do antibiotics work on viral infections like colds or flu?
No. Antibiotics kill bacteria, so they are used to treat bacterial infections. They have no effect on viruses, so they cannot treat viral illnesses such as colds and flu. Unnecessary antibiotic use also increases the risk of antibiotic-resistant bacteria developing.
What is the difference between direct and indirect transmission?
Direct transmission involves close contact between an infected and uninfected person (e.g., touching, sexual intercourse, close-range cough droplets). Indirect transmission uses an intermediate source, such as contaminated food/water, surfaces, or vector organisms (e.g., mosquitoes carrying malaria).
What's Next
Now you have mastered Core content for pathogens, transmission and body defences, you are ready to progress to related subtopics in the Diseases, Immunity and Drugs unit. The next key topic is vaccination, which explains how we get long-term protection against specific pathogens, and is a common 3-4 mark structured question in Core Paper 2. You will also build on this knowledge when studying sexually transmitted infections (including HIV) and the growing global issue of antibiotic resistance. Make sure you practice writing extended responses to defence and transmission questions, as these are often assessed in structured answer sections rather than just multiple choice, and using exact syllabus terminology will help you score full marks.