Syllabus coverage
- Describe a pathogen as a disease-causing organism
- Describe a transmissible disease as a disease in which the pathogen can be passed from one host to another
- State that a pathogen is transmitted by direct contact, including through blood and other body fluids, and indirectly, including from contaminated surfaces, food, animals and air
- Describe the body defences, limited to skin, hairs in the nose, mucus, stomach acid and white blood cells
- Explain the importance of a clean water supply, hygienic food preparation, good personal hygiene, waste disposal and sewage treatment in controlling the spread of disease
- Describe active immunity as defence against a pathogen by antibody production in the body, and explain that active immunity is gained after an infection by a pathogen or by vaccination
- State that each pathogen has its own antigens, which have specific shapes, and that specific antibodies have complementary shapes which fit specific antigens
- Describe antibodies as proteins that bind to antigens, leading to direct destruction of pathogens or marking of pathogens for destruction by phagocytes
- Outline the process of vaccination: weakened pathogens or their antigens are put into the body, the antigens stimulate an immune response by lymphocytes which produce antibodies, and memory cells are produced that give long-term immunity
- Explain the role of vaccination in controlling the spread of diseases, and explain that passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual, including across the placenta and in breast milk
- Explain the importance of breast-feeding for the development of passive immunity in infants, and state that memory cells are not produced in passive immunity
- Describe cholera as a disease caused by a bacterium which is transmitted in contaminated water, and explain that the cholera bacterium produces a toxin that causes secretion of chloride ions into the small intestine, causing osmotic movement of water into the gut, causing diarrhoea, dehydration and loss of ions from the blood
Part of the IGCSE Biology syllabus 0610 / 0970.
Pathogens and transmissible disease
A pathogen is a disease-causing organism. It is not the disease itself - it is the organism that causes it. Cambridge groups pathogens into four types: bacteria, viruses, protoctists and fungi.
A transmissible disease (also called a communicable or infectious disease) is one in which the pathogen can be passed from one host to another. Not every disease is transmissible: sickle-cell anaemia and type 2 diabetes are not caused by a pathogen and cannot be caught from another person.
The four groups differ in size, structure and the way they make you ill. Bacteria are single-celled organisms that reproduce inside the body and often release toxins. Viruses are much smaller and can only reproduce inside a living cell, which they damage. Protoctists are single-celled eukaryotes, and some, such as the malaria parasite, are carried by a vector. Fungi may be single-celled, like yeast, or grow as thread-like hyphae, and they cause diseases such as athlete's foot and ringworm.
| Type of pathogen | Key features | Example disease |
|---|---|---|
| Bacterium | Single-celled organism with no nucleus; reproduces by itself; often produces toxins | Cholera (Vibrio cholerae), tuberculosis |
| Virus | Much smaller than a bacterium and not a cell; can only reproduce inside a host cell, which it damages | Influenza (flu), the common cold, measles |
| Protoctist | Single-celled eukaryotic organism; often spread by a vector | Malaria (Plasmodium, spread by mosquitoes) |
| Fungus | Single-celled or made of thread-like hyphae; often grows on the skin or in warm, damp places | Athlete's foot, ringworm |
How pathogens are transmitted
Pathogens have no way of travelling on their own, so they rely on a route from one host to the next. Cambridge splits these routes into direct contact and indirect transmission.
Direct contact means the pathogen passes straight from one person to another. This includes touching, and contact with blood and other body fluids - which is how HIV, hepatitis and some bacterial infections spread. Indirect transmission means the pathogen travels through something else first: a contaminated surface, food, water, the air, or an animal.
Airborne spread is usually by droplets: when a person coughs or sneezes, tiny droplets carrying pathogens are released and breathed in by someone nearby. Contaminated water and food are a major route for cholera, typhoid and food poisoning. Some pathogens use a vector - an animal that carries the pathogen from one host to another without becoming ill itself. Mosquitoes are the vector for malaria.
| Route | How the pathogen passes | How to reduce the risk |
|---|---|---|
| Direct contact (including blood and body fluids) | Touching an infected person, or contact with their blood or other body fluids | Wash hands; wear gloves; never share needles |
| Contaminated food and water | Eating food or drinking water that carries the pathogen | Clean water supply; hygienic food preparation; sewage treatment |
| Air (droplets) | Breathing in droplets released when an infected person coughs or sneezes | Cover the mouth when coughing; ventilate rooms; keep infected people away from others |
| Vector (for example a mosquito) | An animal carries the pathogen from one host to another | Bed nets, insect repellent, draining standing water, insecticides |
The body's first defences: mechanical and chemical barriers
Before white blood cells get involved, the body has a set of barriers that stop pathogens entering in the first place. Cambridge names four: the skin, the hairs in the nose, mucus and cilia in the trachea and bronchi, and hydrochloric acid in the stomach.
These barriers work in two ways. Some are mechanical - they physically block or trap the pathogen, or sweep it away. Others are chemical - they kill the pathogen or make the environment impossible for it to survive in. The skin and the stomach lining are examples of barriers that act in both ways.
- Skin (mechanical): the outer layer is dead and dry, forming a tough physical barrier; if it is cut, the blood clots and the wound seals.
- Hairs in the nose (mechanical): they trap dust and pathogens in the air you breathe in.
- Mucus (mechanical and chemical): a sticky secretion in the trachea and bronchi that traps pathogens, and contains enzymes that attack bacteria.
- Cilia (mechanical): tiny hair-like structures on the cells lining the airways that beat to sweep mucus, with its trapped pathogens, up towards the throat to be swallowed.
- Hydrochloric acid in the stomach (chemical): it is strongly acidic, so most pathogens swallowed in food or drink are killed before they can reach the intestine.
White blood cells: engulfing, antibodies and antitoxins
If a pathogen gets past the barriers, it meets the white blood cells. Cambridge gives white blood cells three jobs: engulfing pathogens (phagocytosis), producing antibodies, and producing antitoxins.
Phagocytosis is the engulfing of a pathogen. A white blood cell surrounds the pathogen, takes it inside and digests it, destroying it. This is a general defence: it works against any pathogen.
Antibodies are proteins that bind to antigens on the surface of a pathogen. Each pathogen has its own antigens, and each antigen has a specific shape. Antibodies have complementary shapes, so an antibody will only fit the antigen it was made for, like a key fitting one lock. When an antibody binds to an antigen it either destroys the pathogen directly or marks it for destruction by phagocytes.
Some pathogens cause harm by releasing toxins. Antitoxins are proteins produced by white blood cells that bind to these toxins and neutralise them, so the toxin can no longer damage the body's cells.
- Engulfing (phagocytosis): a general defence against any pathogen.
- Antibodies: a specific defence - each antibody fits one antigen.
- Antitoxins: neutralise the toxins released by pathogens.
Active immunity, memory cells and vaccination
Active immunity is defence against a pathogen by antibody production in the body. It is called active because your own immune system has done the work of making the antibodies. It can be gained in two ways: after an infection by a pathogen, or after vaccination.
The first time a particular pathogen infects you, it takes your body some days to make the right antibody, which is why you feel ill. During that response the body also makes memory cells. Memory cells stay in the blood and remember that pathogen. If the same pathogen enters again, the memory cells trigger a much faster and larger production of the correct antibody, often before you feel ill at all. That is why you are immune after an infection.
Vaccination gives you the same immunity without the illness. Weakened, dead or inactive pathogens, or their antigens, are put into the body. They cannot cause the disease, but the antigens they carry stimulate an immune response: lymphocytes produce antibodies, and memory cells are produced that give long-term immunity.
Vaccination protects far more than the person vaccinated. If enough people are immune, the pathogen has nowhere to pass to, so it cannot spread through the population. This is how vaccination controls and can wipe out epidemics: smallpox has been eradicated worldwide, and measles and polio have been pushed back by widespread vaccination.
Passive immunity: the short-term defence
Passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual. The antibodies are not made by the person's own immune system - they are borrowed, ready-made, from someone else.
In humans this happens naturally in two ways. Antibodies pass from a mother to her foetus across the placenta before birth, and a breast-feeding mother passes antibodies to her infant in breast milk. Both give the baby protection during the first months of life, when its own immune system is still developing and it cannot yet make its own antibodies against everything it meets.
Passive immunity is short-term because the borrowed antibodies are gradually broken down, and no memory cells are produced. Once the antibodies are gone, the protection is gone. That is the key difference from active immunity, where memory cells keep the protection going for years.
Cholera, malaria and the control of disease
Cholera is caused by a bacterium, Vibrio cholerae, which is transmitted in contaminated water. The bacterium produces a toxin that causes the secretion of chloride ions into the small intestine. Water then moves into the gut by osmosis, because the gut contents are now more concentrated in ions than the blood. This causes diarrhoea and the loss of water and ions from the blood, leading to severe dehydration. It is treated with rehydration and prevented by a clean water supply, good sanitation and sewage treatment.
Malaria is caused by a protoctist, Plasmodium, and is transmitted by mosquitoes, which act as vectors. The mosquito bites an infected person, picks up the pathogen and injects it into the next person it bites. Malaria is controlled by vector control: sleeping under bed nets, using insect repellent, and draining or spraying standing water where mosquitoes breed. Travellers to affected areas may also take antimalarial tablets, which act as a chemical defence against the pathogen.
Cholera and malaria show why the practical control of disease matters. A clean water supply and sewage treatment stop water-borne diseases spreading; hygienic food preparation and good personal hygiene (washing hands, washing food, keeping surfaces clean) block indirect transmission; and waste disposal removes the places where pathogens and vectors breed. These measures attack the route of transmission, so they protect a whole community, not just one patient.
It is also important to know that bacteria and viruses are different kinds of pathogen. Bacteria are cells and can be killed by antibiotics; viruses are not cells and have no bacterial structures for an antibiotic to attack, so antibiotics do not work against viruses. This is why a doctor refuses antibiotics for a cold or flu.
| Disease | Type of pathogen | Transmission | Control |
|---|---|---|---|
| Cholera | Bacterium (Vibrio cholerae) | Contaminated water (water-borne) | Clean water supply, sewage treatment, rehydration treatment |
| Malaria | Protoctist (Plasmodium) | Mosquito vector | Bed nets, insect repellent, draining standing water, antimalarial tablets |
Key terms you must be able to define
- Pathogen
- A disease-causing organism, such as a bacterium, virus, protoctist or fungus.
- Transmissible disease
- A disease in which the pathogen can be passed from one host to another.
- Vector
- An animal that carries a pathogen from one host to another without becoming ill itself, such as the mosquito that carries malaria.
- Antigen
- A molecule on the surface of a pathogen that has a specific shape; each pathogen has its own antigens, which trigger an immune response.
- Antibody
- A protein produced by lymphocytes that has a shape complementary to a specific antigen; it binds to the antigen, either destroying the pathogen directly or marking it for destruction by phagocytes.
- Phagocytosis
- The engulfing of a pathogen by a white blood cell, which surrounds, takes in and digests the pathogen to destroy it.
- Active immunity
- Defence against a pathogen by antibody production in the body, gained after an infection by the pathogen or by vaccination; memory cells give long-term immunity.
- Passive immunity
- A short-term defence against a pathogen by antibodies acquired from another individual, for example across the placenta or in breast milk; no memory cells are produced.
Applied to the real world
Cambridge sets biology in unfamiliar contexts - a hospital, a farm, a water-treatment plant. Use these as models for how to apply the science, then write your own.
A flood can break a water pipe or overflow a sewage system, so drinking water becomes contaminated with the cholera bacterium. The bacterium is transmitted in contaminated water, so many people in the area drink it and develop cholera at the same time. This is why outbreaks of water-borne disease follow floods and broken water supplies, and why restoring a clean water supply and sewage treatment is the first priority.
A traveller going to a country where malaria is common takes antimalarial tablets and sleeps under a bed net. Malaria is caused by the Plasmodium protoctist and spread by mosquitoes acting as vectors. The tablets are a chemical defence against the pathogen, while the bed net stops the mosquito biting and injecting the pathogen in the first place - vector control.
When a country vaccinates most of its children against a disease such as measles, the vaccine gives each child antibodies and memory cells without causing the illness. If enough people are immune, the pathogen has nowhere to pass to, so it cannot spread. The epidemic is stopped, and diseases such as smallpox have been eradicated worldwide in this way.
A patient with a sore throat asks for antibiotics. If the doctor judges the infection to be viral, such as a cold, the antibiotic is refused. Antibiotics kill bacteria only: a virus is not a cell and has no bacterial structures for the antibiotic to attack. Prescribing it would not treat the illness and would help resistant bacteria to spread.
A newborn baby cannot yet make its own antibodies, so it is vulnerable to infection. Antibodies pass from the mother across the placenta before birth and in breast milk afterwards, giving the baby passive immunity during its first months. The protection is short-term, because the borrowed antibodies are gradually broken down and no memory cells are produced.
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Exam-style questions
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Exam technique for this topic
- Learn the definition of a pathogen word for word - a disease-causing organism - and be ready to name the four groups: bacteria, viruses, protoctists and fungi.
- For transmission questions, state the route and whether it is direct or indirect: direct contact (including blood and other body fluids) or indirect (contaminated surfaces, food, water, air or an animal).
- When asked about body defences, classify each barrier as mechanical or chemical. The skin, nose hairs, mucus and cilia are mechanical; hydrochloric acid in the stomach is chemical.
- Keep active and passive immunity apart. Active immunity makes its own antibodies and memory cells, so it lasts; passive immunity borrows antibodies and produces no memory cells, so it is short-term.
- Give the vaccination process as a chain: weakened, dead or inactive pathogen or its antigens -> antigens stimulate lymphocytes -> antibodies produced -> memory cells give long-term immunity.
- For cholera, give the full mechanism: toxin -> secretion of chloride ions into the small intestine -> osmotic movement of water into the gut -> diarrhoea -> dehydration and loss of ions from the blood.
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Where this fits in IGCSE Biology
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