Infection and responseOxford AQA IGCSE Biology: Revision notes
Section 1
What is a pathogen and how does it cause disease?
A pathogen is a microorganism that enters the body and causes disease. Pathogens are parasitic organisms that damage host cells or produce harmful substances. Not all microorganisms are pathogens; many are harmless or beneficial. When a pathogen successfully invades a host organism, it multiplies and interferes with normal body functions, leading to disease symptoms.
Pathogenic microorganisms are distinct from commensals (harmless organisms) and mutualistic organisms (those that benefit the host). Understanding this distinction is essential for exam success.
Examiners want you to clearly distinguish between a pathogen (a disease-causing microorganism) and a microorganism in general. Always use the term 'pathogen' when describing disease-causing organisms.
Section 2
What are the four main types of pathogens?
Pathogens are classified into four main groups based on their biological structure and characteristics:
| Type | Description | Key Features |
|---|---|---|
| Bacteria | Single-celled prokaryotes | No nucleus; cell wall present; reproduce by binary fission; some produce toxins |
| Viruses | Non-living infectious agents | No nucleus; contain DNA or RNA; reproduce inside host cells only; destroy host cells |
| Fungi | Eukaryotic organisms | Nucleus present; cell wall of chitin; reproduce via spores; cause infections of skin and nails |
| Protists | Single-celled eukaryotes | Nucleus present; often motile; many are parasitic; transmitted by vectors |
Each type has distinct structural features that determine how it infects the body and causes disease. Understanding these differences is crucial for explaining disease transmission and infection mechanisms.
Students often confuse viruses as 'living' or 'cells'. Viruses are not cells and are not living in the conventional sense—they cannot reproduce, obtain energy, or grow independently.
Think of viruses as 'biological pirates'—they hijack the host cell's machinery to make copies of themselves, rather than reproducing on their own like bacteria do.
Section 3
How are pathogens transmitted between hosts?
Pathogens spread from one infected individual to another or from environmental sources through several transmission routes:
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Direct contact: Pathogens pass from infected to uninfected individuals through physical touching, kissing, or sexual contact. Examples: bacterial skin infections, HIV transmission through bodily fluids.
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Water transmission: Pathogens in contaminated drinking water or water sources cause infection when ingested. Example: cholera bacteria spread through contaminated water supplies.
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Air transmission: Pathogens released in respiratory droplets from coughing, sneezing, or breathing spread to nearby individuals who inhale them. Example: tuberculosis bacteria spread through airborne droplets.
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Vector transmission: Organisms (vectors) such as mosquitoes, ticks, or flies carry and transmit pathogens. The vector is infected and transfers the pathogen when it bites or feeds. Example: malaria protist transmitted by mosquitoes.
Understanding transmission routes is essential for explaining disease spread and public health prevention strategies.
When answering exam questions about disease spread, always name the specific transmission route and explain how the pathogen enters the new host's body—simply stating 'it spreads' is insufficient.
If asked how tuberculosis spreads: State that tuberculosis bacteria are transmitted via air transmission. Explain that bacteria are released in respiratory droplets when an infected person coughs or sneezes, and nearby individuals inhale these droplets, allowing bacteria to enter their lungs.
Section 4
What are examples of diseases caused by different pathogen types?
The major pathogenic diseases specified by Oxford AQA are:
| Disease | Pathogen Type | Description |
|---|---|---|
| Tuberculosis (TB) | Bacteria (Mycobacterium tuberculosis) | Infection of the lungs causing persistent cough, chest pain, and tissue damage; spread by air transmission |
| HIV/AIDS | Virus (Human Immunodeficiency Virus) | Attacks the immune system, particularly T lymphocytes; weakens immune response; spread by direct contact with bodily fluids |
| Ringworm | Fungus (Trichophyton species) | Fungal skin infection causing circular red patches on skin; spread by direct contact; not caused by a worm |
| Malaria | Protist (Plasmodium species) | Parasitic infection causing fever, chills, and anaemia; spread by vector transmission (mosquitoes) |
Each disease demonstrates distinct characteristics of its pathogen type and a different mode of transmission.
Students sometimes think ringworm is caused by a parasitic worm. It is actually caused by a fungus and has nothing to do with worms—the name refers to the ring-shaped appearance of the infection.
In exam questions, always link the disease to the correct pathogen type and specify the transmission route. For example, do not just say 'malaria is spread by mosquitoes'—state that 'malaria protists are transmitted by mosquito vectors'.
Section 5
How do bacteria cause disease?
Bacteria damage the host organism through two primary mechanisms:
1. Production of toxins
- Many pathogenic bacteria produce bacterial toxins (exotoxins or endotoxins) that poison host cells
- Toxins disrupt cellular functions, damage tissues, and cause symptoms of disease
- Example: Vibrio cholerae produces enterotoxin that damages intestinal cells, causing severe diarrhoea
- The bacterial cell does not need to enter the host cell to cause damage; toxins diffuse into the bloodstream and affect distant tissues
2. Damaging cells directly
- Bacteria invade and destroy host cells through enzymatic breakdown or physical damage
- When bacteria rupture through the cell membrane to escape, they cause cell lysis (cell death)
- Example: Mycobacterium tuberculosis invades lung cells and destroys lung tissue, impairing gas exchange
- Accumulated damage from millions of bacteria leads to tissue destruction and organ dysfunction
Bacterial diseases are often treatable with antibiotics, which target bacterial structures (e.g., cell walls) or processes (e.g., protein synthesis) without harming host cells.
Examiners expect you to explain both mechanisms: toxin production (which can happen without direct cell invasion) and direct cell damage. A complete answer should address both pathways.
How does Vibrio cholerae cause disease? It produces cholera toxin that damages intestinal epithelial cells, disrupting ion and water balance. This causes the severe diarrhoea characteristic of cholera, even though the bacteria themselves may remain in the intestinal lumen.
Section 6
How do viruses cause disease?
Viruses cause disease through a distinctive infection mechanism that differs fundamentally from bacteria:
1. Entering host cells
- Viral particles recognise and attach to specific receptor molecules on the surface of host cells
- The virus injects its genetic material (DNA or RNA) into the host cell or is engulfed by endocytosis
- Once inside, the virus hijacks the cell's ribosomes and metabolic machinery
2. Reproducing inside cells
- The viral genetic material uses host cell enzymes and ribosomes to synthesise viral proteins
- Multiple copies of viral nucleic acid and proteins are assembled into new viral particles
- New viruses are released by budding through the cell membrane or by bursting out (lysis) of the cell
3. Damaging or destroying cells
- Viral reproduction consumes host cell resources, starving the cell of essential molecules
- The rupture of the cell membrane during viral release causes cell lysis and cell death
- Immune response to infected cells may also trigger apoptosis (programmed cell death)
- Large-scale destruction of infected cells leads to tissue damage and disease symptoms
- Example: HIV destroys T lymphocytes, progressively weakening the immune system; influenza viruses destroy respiratory epithelial cells, causing coughing and breathing difficulty
Viral diseases are difficult to treat because viruses use host cell machinery; antivirals are less selective than antibiotics.
Students sometimes say viruses 'eat' cells or 'poison' them like bacteria do. Viruses do not produce toxins; they cause damage by reproducing inside cells and destroying them from within.
Viruses are like 'cellular parasites' that move into a host cell, use all the cell's resources to make copies of themselves, and then destroy the house (cell) when they leave.
Explain how HIV causes disease: HIV enters T lymphocytes by binding to receptor molecules. Inside the cell, it reproduces, creating new viral particles. These new viruses exit by budding, damaging the cell membrane and killing the T lymphocyte. As millions of T cells are destroyed, the immune system weakens, leading to AIDS.
Must Know
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A pathogen is a microorganism that causes disease; the four main types are bacteria (prokaryotes), viruses (non-living; reproduce only inside cells), fungi (eukaryotes with chitin cell walls), and protists (single-celled eukaryotes, often parasitic).
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Transmission routes: Direct contact (physical touching), water transmission (ingestion of contaminated water), air transmission (inhalation of respiratory droplets), and vector transmission (spread by organisms such as mosquitoes).
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Key diseases: Tuberculosis (bacterium; air transmission); HIV/AIDS (virus; direct contact with bodily fluids); ringworm (fungus; direct contact); malaria (protist; mosquito vector).
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Bacteria cause disease by producing toxins that poison host cells (without invading them) and by damaging cells directly through invasion and lysis, leading to tissue destruction.
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Viruses cause disease by entering host cells, using the cell's machinery to reproduce, and destroying cells during viral release (lysis) or triggering apoptosis; large-scale cell death causes disease symptoms.
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For exam success, always specify the pathogen type, describe the transmission route, and explain the mechanism of cellular damage or toxin production.
That's the notes covered.
Carry on to the next subtopic.