Understanding Diseases and immunity: IGCSE Biology Explained

Your body is under constant siege

Every surface you touch, every breath you take and every mouthful you swallow brings you into contact with microorganisms. The vast majority are harmless. Some are beneficial. A small fraction, however, are pathogens: organisms that cause disease. Understanding how pathogens operate, how the body defends itself and how medicine intervenes through vaccination forms one of the most heavily examined sections of the Cambridge IGCSE Biology syllabus.

Pathogens and transmissible diseases

A pathogen is a disease-causing organism. Pathogens include bacteria, viruses and fungi. Each type causes disease in a different way: bacteria may release toxins that damage tissues; viruses invade host cells and hijack their machinery to replicate; pathogenic fungi may digest living tissue or release spores that trigger infection.

A transmissible disease (also called an infectious or communicable disease) is one in which the pathogen can be passed from one host to another. Not all diseases are transmissible. Cancer, for instance, is not caused by a pathogen and cannot spread between individuals. The distinction matters in the exam: if the question asks whether a disease is transmissible, the answer depends entirely on whether a pathogen is involved and whether it can transfer between hosts.

How pathogens are transmitted

Transmission routes fall into two broad categories.

• Direct contact: the pathogen passes physically from one person to another. This includes contact with infected blood or other body fluids, skin-to-skin contact and sexual transmission. HIV, for example, spreads through blood and body fluids.
• Indirect contact: the pathogen reaches a new host without direct physical contact with the infected individual. Routes include contaminated surfaces (doorknobs, shared equipment), contaminated food or water (cholera, food poisoning), airborne droplets (influenza, tuberculosis) and animal vectors (mosquitoes transmitting malaria).

The body's defences

Before a pathogen can cause disease, it must first breach the body's physical and chemical barriers. The IGCSE syllabus organises these defences into two broad tiers: non-specific barriers that work against all pathogens regardless of type, and white blood cells that mount a targeted response.

Extended: Two types of white blood cell response

The Cambridge IGCSE syllabus requires Supplement students to distinguish between two categories of white blood cell action.

• Phagocytes carry out phagocytosis. They engulf pathogens by surrounding them with their cell membrane, drawing them into the cell and digesting them with enzymes. Phagocytes are non-specific: they attack any foreign particle they encounter, regardless of the type of pathogen.
• Lymphocytes produce antibodies. Each type of pathogen carries specific molecules on its surface called antigens. Lymphocytes recognise these antigens and produce antibodies that are complementary in shape to the antigen. The antibodies bind to the antigens, which may clump the pathogens together, neutralise toxins or mark the pathogen for destruction by phagocytes. This response is highly specific: antibodies produced against one pathogen will not work against a different one.

Controlling the spread of disease

Public health measures reduce the transmission of pathogens at the population level. The syllabus identifies five key strategies.

1. A clean water supply: treating and filtering drinking water removes waterborne pathogens such as the bacterium that causes cholera. In regions without clean water, diarrhoeal diseases remain a leading cause of death.
2. Hygienic food preparation: cooking food to a high temperature kills most bacteria. Keeping raw and cooked food separate prevents cross-contamination. Proper refrigeration slows bacterial reproduction.
3. Good personal hygiene: regular handwashing with soap removes pathogens from the skin before they can enter the body through the mouth, nose or eyes.
4. Waste disposal: safe collection and disposal of household and industrial waste prevents the accumulation of materials in which pathogens can breed.
5. Sewage treatment: processing human waste before it enters waterways prevents faecal pathogens from contaminating drinking water or food sources. The syllabus does not require details of treatment stages, but you should know that the purpose is to kill pathogens and remove harmful substances.

Immunity

Immunity is the ability to resist infection by a pathogen. An immune individual either prevents the pathogen from establishing itself or destroys it so rapidly that no symptoms develop. The IGCSE syllabus distinguishes between two forms of immunity.

How vaccination works

Vaccination is the most important medical application of active immunity. The process exploits the immune system's ability to remember a pathogen it has encountered before.

1. A vaccine containing a weakened, dead or inactivated form of the pathogen (or its antigens) is introduced into the body, usually by injection.
2. The antigens on the surface of the weakened pathogen stimulate lymphocytes to produce specific antibodies.
3. The lymphocytes also produce memory cells that remain in the blood long after the initial immune response has subsided.
4. If the same live pathogen enters the body in the future, the memory cells recognise its antigens immediately and trigger a rapid, large-scale production of the correct antibodies.
5. The pathogen is destroyed before it can multiply enough to cause symptoms. The individual is immune.

The vaccine itself does not cause the disease because the pathogen it contains is dead, weakened or fragmented. It cannot reproduce or damage tissues. It serves only to present antigens to the immune system.

Extended: Why vaccination matters at the population level

When a large proportion of a population is vaccinated against a disease, the pathogen finds it difficult to spread because most potential hosts are immune. This concept, sometimes called herd immunity, protects individuals who cannot be vaccinated, such as very young infants, elderly people or those with compromised immune systems. The pathogen effectively runs out of susceptible hosts and the chain of transmission is broken.

This is why public health authorities set vaccination coverage targets. If the proportion of immune individuals drops below a critical threshold, outbreaks can re-emerge even in communities that were previously disease-free. Measles outbreaks in recent years have illustrated this principle with considerable clarity.

Common exam mistakes

1. Confusing antibodies with antibiotics. Antibodies are proteins produced by lymphocytes that target specific antigens on pathogens. Antibiotics are drugs (such as penicillin) that kill bacteria or inhibit their growth. Antibiotics have no effect on viruses. This distinction appears frequently in exam questions and confusing the two terms will cost marks.
2. Writing that vaccines "cure" disease. Vaccines prevent disease by stimulating the immune system before infection occurs. They are not treatments. A person who already has measles cannot be cured by receiving the measles vaccine.
3. Stating that white blood cells "eat" pathogens. The precise term is phagocytosis. Phagocytes engulf and digest pathogens. Using "eat" in an exam answer is too informal and may not earn the mark.
4. Forgetting memory cells when explaining vaccination. The entire point of vaccination is to generate memory cells. Without mentioning them, your explanation of why vaccination provides long-lasting immunity is incomplete.
5. Saying passive immunity is "weaker" than active immunity. Passive immunity is not weaker in terms of the antibodies themselves. It is shorter-lived because the body does not produce memory cells. The antibodies provided are effective but are gradually broken down.
6. Confusing "transmissible" with "contagious." For the purposes of the IGCSE exam, transmissible means the pathogen can pass from one host to another by any route, including vectors. Focus on the definition: a pathogen is involved and it can transfer between hosts.

Self-check questions

1. Define the term "pathogen" and give one example each of a bacterial, viral and fungal pathogen.
2. Distinguish between direct and indirect transmission of disease, giving one named example of each.
3. Explain how stomach acid and mucus in the airways each help to prevent infection.
4. Extended: Describe the difference between the roles of phagocytes and lymphocytes in defending the body against pathogens.
5. Explain, step by step, how vaccination provides long-term protection against a specific disease.
6. A newborn baby receives antibodies from its mother through breast milk. State whether this is an example of active or passive immunity, and explain why.
7. Explain why antibiotics are effective against bacterial infections but not against viral infections.