Understanding Disease: Edexcel IGCSE Human Biology Explained

Disease is one of those topics where the facts are straightforward but the detail matters. Here is what the Edexcel IGCSE Human Biology specification actually requires, stripped to the essentials.

The edexcel igcse human biology disease section of the course covers two areas: pathogens and disease transmission, plus immunity and public health. Both appear regularly on the exam, and the questions tend to reward precise, no-nonsense answers. Vague statements about germs will not earn marks. Specific organisms, specific transmission routes and specific immune responses will. The edexcel igcse human biology revision notes below give you exactly what you need.

The general course of a disease

Every infectious disease follows a predictable sequence:

Infection - the pathogen enters the body through a route of transmission (e.g. airborne droplets, contaminated water, direct contact, vectors)
Incubation - the pathogen reproduces inside the body. The infected person shows no symptoms yet, but may be contagious
Symptoms - the body's immune response and/or damage caused by the pathogen produces visible signs of illness (fever, rash, diarrhoea, etc.)

The Edexcel IGCSE exam expects you to state this sequence precisely. "Infection, incubation, symptoms" is the order. Mixing them up loses marks.

Types of pathogens

Viruses

Viruses are not cells. They are particles made of a protein coat surrounding a core of genetic material (DNA or RNA). They are much smaller than bacteria and can only reproduce inside a living host cell. The virus injects its genetic material into the host cell, hijacks the cell's machinery to make copies of itself, and the new viruses burst out, destroying the host cell. This is why antibiotics do not work against viruses: antibiotics target cell processes that viruses do not have.

Bacteria

Bacteria are single-celled organisms with a cell wall, cell membrane, cytoplasm and DNA that floats freely in the cytoplasm (no nucleus). Some bacteria also have a flagellum for movement and plasmids (small rings of extra DNA). Bacteria can reproduce rapidly by binary fission, doubling their population every 20 minutes under ideal conditions. Not all bacteria are harmful; many are essential for decomposition and nutrient cycling.

Bacterial growth curves follow a characteristic pattern: a lag phase (bacteria adapt to new conditions), a log phase (exponential growth), a stationary phase (growth rate equals death rate) and a decline phase (nutrients run out, waste accumulates). Exam questions may present a growth curve and ask you to identify or explain each phase.

Fungi

The specification covers one fungal disease: athlete's foot, caused by a fungus that thrives in warm, moist conditions.

Specific diseases: transmission, treatment and prevention

The exam tests knowledge of specific diseases. Here is the information the mark scheme requires for each:

Disease	Pathogen	Transmission	Treatment	Prevention
Cholera	Bacterium (Vibrio cholerae)	Contaminated water or food	Oral rehydration therapy (ORT); antibiotics in severe cases	Clean water supplies; proper sewage treatment; good hygiene
Gonorrhoea	Bacterium (Neisseria gonorrhoeae)	Sexual contact	Antibiotics	Use of condoms; limiting sexual partners
HIV/AIDS	Virus (human immunodeficiency virus)	Exchange of body fluids: unprotected sexual contact, sharing needles, mother to child during birth or breastfeeding	Antiretroviral drugs (control but do not cure)	Use of condoms; screening blood donations; needle exchange programmes; antiretroviral drugs during pregnancy
Ebola	Virus	Direct contact with body fluids of infected person	Supportive care (fluids, oxygen); experimental treatments	Isolation of patients; protective equipment for healthcare workers; safe burial practices
Athlete's foot	Fungus	Direct contact with infected skin or surfaces (e.g. shower floors)	Antifungal creams or powders	Keeping feet dry; wearing sandals in communal areas; not sharing towels
Malaria	Protist (Plasmodium)	Vector: female Anopheles mosquito injects protist while feeding on blood	Antimalarial drugs	Mosquito nets; insecticides; draining stagnant water; antimalarial drugs as prophylaxis
Typhoid	Bacterium (Salmonella typhi)	Vector: housefly transfers bacteria from contaminated matter to food; also contaminated water	Antibiotics	Clean water; proper sanitation; vaccination; food hygiene

Exam tip: Oral rehydration therapy (ORT) is a solution of water, salts and sugar. It does not kill the pathogen. It replaces the water and electrolytes lost through diarrhoea, preventing dehydration while the body's immune system fights the infection. Mark schemes penalise answers that say ORT "cures" the disease.

Vectors: mosquitoes and houseflies

A vector is an organism that transmits a pathogen from one host to another without being affected itself. The female Anopheles mosquito is the vector for malaria: when it bites an infected person, it takes up Plasmodium parasites with the blood. When it bites another person, it injects the parasites with its saliva. The housefly is the vector for typhoid: it lands on contaminated material (faeces, rubbish), picks up bacteria on its body and legs, and transfers them to food or surfaces.

The immune response

White blood cells defend the body against pathogens through two main mechanisms:

Phagocytosis - phagocytes (a type of white blood cell) engulf and digest pathogens. This is a non-specific response: phagocytes attack any foreign particle they encounter.
Antibody production - lymphocytes (another type of white blood cell) recognise specific antigens on the surface of a pathogen. They produce antibodies that are complementary to the antigen. Antibodies bind to antigens, clumping pathogens together and marking them for destruction. This is a specific response: each antibody matches only one type of antigen.

The antibody-antigen reaction is lock-and-key specific. Once a lymphocyte has been activated by a particular antigen, some of them become memory cells. If the same pathogen enters the body again, memory cells respond much faster and produce antibodies in larger quantities, destroying the pathogen before symptoms develop. This is the basis of immunity.

Types of immunity

Type	How acquired	Duration	Example
Natural active	Recovering from an infection; memory cells produced	Long-lasting (often lifelong)	Catching chickenpox and becoming immune
Artificial active	Vaccination; weakened or dead pathogen injected to trigger antibody production	Long-lasting	MMR vaccine
Natural passive	Antibodies passed from mother to baby (across placenta or in breast milk)	Short-term (antibodies break down)	Newborn's immunity in first few months
Artificial passive	Injection of antibodies produced by another organism	Short-term	Anti-venom after a snake bite

Exam tip: The distinction between active and passive immunity hinges on one question: does the person's own immune system produce antibodies? If yes, it is active. If antibodies are received from an external source, it is passive. Active immunity is long-lasting because memory cells are formed. Passive immunity is temporary because the received antibodies degrade over time and no memory cells are made.

Vaccines

Vaccines contain a weakened, dead or fragment form of a pathogen. When injected, the antigens trigger the immune system to produce antibodies and memory cells, without causing the actual disease. If the vaccinated person later encounters the live pathogen, the memory cells mount a rapid secondary immune response, producing antibodies quickly enough to prevent illness.

Vaccination programmes aim for herd immunity: if enough of the population is vaccinated, the pathogen cannot spread easily, protecting those who cannot be vaccinated (e.g. people with weakened immune systems, very young infants).

Antibiotics and antibiotic resistance

Antibiotics are substances produced by microorganisms (originally fungi such as Penicillium) that kill or inhibit the growth of bacteria. They target bacterial processes such as cell wall synthesis, which is why they work against bacteria but not against viruses (viruses lack cell walls and use the host cell's machinery).

MRSA and antibiotic resistance

Antibiotic resistance arises through natural selection:

Within a population of bacteria, random mutations may produce individuals that are resistant to a particular antibiotic.
When the antibiotic is used, non-resistant bacteria are killed, but resistant individuals survive.
The resistant bacteria reproduce rapidly, passing the resistance gene to their offspring.
Over time, the resistant strain becomes dominant in the population.

MRSA (methicillin-resistant Staphylococcus aureus) is a bacterium resistant to several common antibiotics. It is a particular concern in hospitals, where patients with weakened immune systems are vulnerable and antibiotics are used frequently. Preventing MRSA involves strict hygiene (handwashing, surface cleaning), completing full courses of antibiotics, and not using antibiotics unnecessarily.

Worked example: antibiotic resistance

Question: Explain how a population of bacteria can become resistant to an antibiotic. (4 marks)

Model answer: A random mutation in one bacterium produces resistance to the antibiotic (1). When the antibiotic is applied, non-resistant bacteria are killed (1). The resistant bacterium survives and reproduces, passing the resistance allele to its offspring (1). Over time, the proportion of resistant bacteria in the population increases (1).

Investigating antibacterial agents

The specification requires practical knowledge of testing antibacterial agents and antibiotics. A typical method involves spreading bacteria on an agar plate, placing paper discs soaked in different antibacterial agents on the plate, incubating at 25 degrees Celsius (not 37 degrees, to reduce the risk of growing human pathogens), and measuring the clear zones (zones of inhibition) around each disc. A larger clear zone indicates a more effective agent. Aseptic technique must be used throughout to prevent contamination.

Decomposers

Non-pathogenic bacteria and fungi play a vital role as decomposers. They break down dead organic matter and waste, recycling nutrients back into the soil. Without decomposers, dead material would accumulate and essential elements like carbon and nitrogen would be locked away from living organisms.

Sewage treatment

The specification covers sewage treatment in modern sewage works and in a pit latrine.

In modern sewage works, the process involves several stages:

Screening - large debris is removed
Primary treatment - sewage is held in settlement tanks where solid matter sinks to the bottom as sludge
Secondary treatment - the liquid is passed through aeration tanks or trickling filter beds, where aerobic microorganisms break down dissolved organic matter
Sludge treatment - sludge is digested by anaerobic microorganisms in sealed tanks, producing methane (which can be used as fuel) and a residue that can be used as fertiliser
Final treatment - the treated water may be further purified before being discharged into rivers or the sea

A pit latrine is simpler: waste is deposited into a deep pit, where anaerobic bacteria decompose it over time. Pit latrines are common where modern sewage infrastructure is unavailable. They must be sited away from water sources to prevent contamination.

Linking disease topics to exam questions

Disease edexcel igcse questions fall into predictable patterns. You might be asked to compare the structure of a virus and a bacterium, explain how a specific disease is transmitted and prevented, describe the immune response to a pathogen, or explain why antibiotics do not work against viral infections. The strongest answers use precise biological vocabulary and give specific examples rather than general statements.

The edexcel igcse human biology practice questions on disease reward directness. State the pathogen. Name the transmission route. Describe the prevention method. If the question asks "explain," give a reason. If it asks "describe," give facts. The command word tells you how much depth to provide, and matching your answer to it is a skill worth practising on IGCSE 4HB1 disease past papers.

Self-check questions

Test your knowledge of the disease edexcel igcse human biology notes below. Attempt each question without looking at your revision materials first.

State the three stages in the general course of a disease.
Describe two structural differences between a virus and a bacterium.
Explain the role of the mosquito in the transmission of malaria.
Distinguish between phagocytosis and antibody production as defence mechanisms.
A vaccine contains weakened pathogens. Explain how vaccination leads to immunity without causing the disease.
Compare natural active immunity with artificial passive immunity. State one similarity and two differences.
Explain why the overuse of antibiotics has led to the emergence of antibiotic-resistant bacteria such as MRSA.
Describe two roles of aerobic microorganisms in modern sewage treatment.

Work through these systematically and then check each answer against the mark scheme. For more edexcel igcse human biology explained content organised by topic, the Green Bridge CBT platform provides structured revision with instant feedback on your answers. Understanding disease transmission, immunity and public health is not only valuable for the exam but connects directly to real-world health decisions that matter well beyond the classroom.