What Is Pathogens And Disease Transmission? A Visual Guide for Edexcel IGCSE Human Biology Students

Pathogens and disease transmission is among the most commonly examined areas of the Edexcel IGCSE Human Biology specification. Precision with terminology and specific disease knowledge are what separate strong answers from weak ones.

Key facts

A pathogen is a microorganism that causes disease
Pathogens include viruses, bacteria, fungi and protists
Disease follows a consistent course: infection, incubation, then symptoms
Transmission routes include direct contact, airborne droplets, contaminated water or food, body fluids and vectors
A vector is an organism that transmits a pathogen without being affected itself
The body defends itself through physical barriers, phagocytosis and antibody production

The general course of disease

Every infectious disease progresses through three stages in a fixed sequence. The Edexcel IGCSE exam expects this sequence to be stated precisely:

Infection - the pathogen enters the body through a specific route (respiratory tract, broken skin, ingestion, mucous membranes or via a vector)
Incubation - the pathogen reproduces inside the body. The host shows no symptoms during this period but may be capable of transmitting the pathogen to others
Symptoms - the damage caused by the pathogen and/or the body's immune response produces observable signs of illness

Viruses: structure and reproduction

Viruses are not classified as living organisms because they cannot carry out metabolic processes independently. A virus particle consists of a core of genetic material (DNA or RNA) surrounded by a protein coat (capsid). Some viruses also have an outer lipid envelope. Viruses are much smaller than bacteria and can only be seen with an electron microscope.

Viral reproduction follows a characteristic pattern:

The virus attaches to a specific receptor on the host cell surface
The virus injects its genetic material into the host cell (or the entire virus is engulfed)
The viral genetic material hijacks the host cell's machinery, directing it to produce copies of the viral DNA/RNA and viral proteins
New virus particles are assembled inside the host cell
The host cell bursts (lysis), releasing the new viruses to infect other cells

This is why antibiotics do not work against viruses. Antibiotics target specific features of bacterial cells (such as cell wall synthesis), and viruses lack these structures entirely.

Bacteria: structure, nutrition and reproduction

Bacteria are single-celled prokaryotic organisms. Their structure includes:

Cell wall (provides structural support and protection)
Cell membrane (controls what enters and leaves the cell)
Cytoplasm (where metabolic reactions occur)
Circular DNA (not enclosed in a nucleus; floats freely in the cytoplasm)
Plasmids (small, additional rings of DNA; may carry genes for antibiotic resistance)
Flagellum (in some species; used for movement)

Bacteria reproduce asexually by binary fission. Under optimal conditions (adequate nutrients, warmth, moisture), a bacterial population can double every 20 minutes. This rapid reproduction is what makes bacterial infections potentially dangerous if left untreated.

Bacterial growth curves

When bacteria are cultured in a closed system, the population follows a predictable growth curve with four phases:

Phase	What happens
Lag phase	Bacteria adapt to the new environment. Population size remains roughly constant as cells prepare for division.
Log (exponential) phase	Conditions are optimal. Bacteria divide rapidly by binary fission. Population doubles at regular intervals.
Stationary phase	The rate of reproduction equals the rate of death. Nutrients are depleted and waste products accumulate. Population size remains stable.
Decline (death) phase	Death rate exceeds reproduction rate. Nutrients are exhausted and toxic waste levels are high. Population decreases.

Exam questions may present a growth curve graph and ask you to identify a phase, explain what is happening, or predict what would happen if a variable were changed (e.g. adding fresh nutrients during the stationary phase would trigger a new log phase).

Specific diseases: what the specification requires

The edexcel igcse human biology explained approach to disease questions is to know four things for each named disease: the causative pathogen, the method of transmission, the treatment and the prevention. Here is the full set:

Viral diseases

HIV/AIDS - caused by the human immunodeficiency virus. Transmitted through exchange of body fluids: unprotected sexual contact, sharing contaminated needles, or from mother to child during birth or breastfeeding. HIV attacks lymphocytes, weakening the immune system until it can no longer defend against opportunistic infections (this stage is called AIDS). Treatment: antiretroviral drugs slow viral replication but do not cure the infection. Prevention: condom use, needle exchange programmes, screening blood donations, antiretroviral therapy during pregnancy.

Ebola - caused by the Ebola virus. Transmitted through direct contact with body fluids (blood, vomit, sweat) of an infected person, or with contaminated surfaces. Symptoms include fever, internal and external bleeding and organ failure. Treatment: supportive care (intravenous fluids, oxygen, blood pressure management); experimental antiviral treatments exist. Prevention: isolation of patients, personal protective equipment for healthcare workers, safe burial practices.

Bacterial diseases

Cholera - caused by Vibrio cholerae. Transmitted through contaminated water or food (faecal-oral route). Causes severe watery diarrhoea, leading to rapid dehydration. Treatment: oral rehydration therapy (ORT), a solution of water, salts and sugar that replaces lost fluids and electrolytes; antibiotics in severe cases. Prevention: clean water supplies, proper sewage treatment, hand washing, food hygiene.

Exam tip: ORT does not kill the pathogen. It prevents death from dehydration while the immune system fights the infection. This distinction is tested directly, and stating that ORT "cures" cholera will lose the mark.

Gonorrhoea - caused by Neisseria gonorrhoeae. Transmitted through sexual contact. Symptoms include discharge and pain during urination. Treatment: antibiotics (though resistant strains are increasing). Prevention: condom use, limiting sexual partners.

Fungal diseases

Athlete's foot - caused by a fungus that thrives in warm, moist conditions. Transmitted by direct contact with infected skin or contaminated surfaces (shower floors, towels). Symptoms include itchy, flaking, cracked skin between the toes. Treatment: antifungal creams or powders. Prevention: keeping feet clean and dry, wearing sandals in communal wet areas, avoiding sharing towels.

Vector-borne diseases

Malaria - caused by the protist Plasmodium. Transmitted by the female Anopheles mosquito, which acts as a vector. When the mosquito feeds on the blood of an infected person, it ingests Plasmodium parasites. When it subsequently bites another person, the parasites are injected with the mosquito's saliva. Symptoms include recurring fever, chills and sweating. Treatment: antimalarial drugs. Prevention: insecticide-treated mosquito nets, indoor residual spraying, draining stagnant water to eliminate mosquito breeding sites, prophylactic antimalarial drugs for travellers.

Typhoid - caused by Salmonella typhi. Transmitted by the housefly, which acts as a vector by landing on contaminated material (faeces, decaying food) and transferring bacteria to food or surfaces via its body and legs. Also transmitted through contaminated water. Symptoms include high fever, abdominal pain and diarrhoea. Treatment: antibiotics. Prevention: clean water supply, proper sanitation, vaccination, food hygiene.

The body's defences against pathogens

The immune system operates through non-specific and specific defences:

Non-specific defences (first line)

Skin acts as a physical barrier
Mucus in the airways traps pathogens
Stomach acid (hydrochloric acid) kills many ingested pathogens
Phagocytes engulf and digest pathogens regardless of type (non-specific)

Specific immune response

Lymphocytes mount a targeted response to specific pathogens:

Each pathogen carries unique antigens on its surface
A lymphocyte with a complementary antibody recognises the antigen
The lymphocyte is activated and produces large quantities of specific antibodies
Antibodies bind to the antigens, clumping pathogens together (agglutination) and marking them for destruction by phagocytes
Some activated lymphocytes become memory cells, which remain in the body and enable a faster, stronger response if the same pathogen enters again

Exam tip: When explaining antibody production, always state that antibodies are specific to a particular antigen. This specificity is what makes the immune response targeted. A generic answer that says "white blood cells produce antibodies" without mentioning antigen specificity will not earn full marks.

Worked example: immune response

Question: Explain how the body's immune system responds to a bacterial infection. (4 marks)

Model answer: Phagocytes engulf and digest the bacteria (1). Lymphocytes recognise the specific antigens on the bacterial surface (1). The lymphocytes produce antibodies that are complementary to the bacterial antigens (1). Some lymphocytes become memory cells, enabling a faster response if the same bacteria enter the body again (1).

Investigating antibacterial agents

The practical investigation of antibacterial agents involves culturing bacteria on agar plates and placing paper discs soaked in different agents (e.g. antibiotics, disinfectants, antiseptics) on the surface. After incubation at 25 degrees Celsius (to avoid growing human pathogens at body temperature), clear zones (zones of inhibition) appear around effective agents. A larger zone indicates a more effective agent. Aseptic technique (sterilising equipment, working near a Bunsen flame, sealing plates) is essential to prevent contamination and ensure valid results.

Exam question patterns for pathogens and disease transmission

Pathogens and disease transmission explained in the exam follows consistent patterns. You may be asked to compare the structure of viruses and bacteria, describe how a named disease is transmitted and prevented, explain the role of a vector, describe how the immune system responds to a pathogen, or explain why antibiotics are ineffective against viral infections. The key to strong answers is specificity. Name the pathogen. Name the transmission route. Name the vector. Use the precise biological terms the edexcel igcse human biology definition of each concept requires.

Self-check questions

Use these to test your knowledge of what is pathogens and disease transmission igcse content at exam level:

State the three stages in the general course of a disease and briefly describe each.
Draw and label a diagram of a bacterial cell. Include at least five structures.
Explain why antibiotics kill bacteria but do not affect viruses.
Describe the role of the female Anopheles mosquito in the transmission of malaria.
A person has been vaccinated against measles. They are later exposed to the measles virus. Explain why they do not develop the disease.
Identify the four phases of a bacterial growth curve and explain what happens during the stationary phase.
Compare the transmission and prevention of cholera and HIV.
Describe one method used to investigate the effectiveness of different antibiotics on bacterial growth.

For additional edexcel igcse pathogens and disease transmission resources and edexcel igcse human biology notes organised by topic, the Green Bridge CBT platform offers structured 4HB1 revision with instant feedback. Understanding the mechanisms of infection and immunity provides a foundation that extends well beyond the exam into fields such as medicine, epidemiology and public health.