Topic D: Health and Disease

This part applies biological knowledge to personal and public health. You consider what a healthy diet and lifestyle involve, then study pathogens and how diseases spread, the body's lines of defence including the immune response and immunisation, and finally non-infectious diseases such as cancer and the effects of drugs. Exam questions often present a scenario or data and ask you to explain causes, transmission or prevention, so applying principles clearly is more important than recall alone.

Diet, health and lifestyle

Good health depends on a balanced diet, regular exercise and avoidance of harmful habits. The diet must supply energy and nutrients in proportions suited to age, sex and activity. Too much energy-rich food, especially fat and sugar, with too little exercise leads to obesity, which increases the risk of heart disease, diabetes and joint problems. Too little of a nutrient causes deficiency diseases, for example scurvy from lack of vitamin C and anaemia from lack of iron. Dietary fibre keeps the gut working and helps prevent constipation. You should be able to interpret food-energy data, calculate energy intake against requirement, and recommend dietary changes for a given individual, justifying each suggestion biologically.

Pathogens and infectious disease

An infectious disease is caused by a pathogen, a disease-causing microorganism such as a bacterium, virus, fungus or protozoan, that can be passed from one host to another. Bacteria may release toxins that damage tissues, while viruses invade cells and take over their machinery to reproduce, destroying the cell. Examples include tuberculosis and food poisoning caused by bacteria, influenza and AIDS caused by viruses, and athlete's foot caused by a fungus. You should be able to name the type of pathogen responsible for common diseases and explain how it harms the body, distinguishing infectious diseases, which can spread, from non-infectious diseases, which cannot.

Transmission and prevention

Pathogens spread by several routes: through the air in droplets when an infected person coughs or sneezes, through contaminated food and water, by direct contact, through body fluids such as blood, and by vectors such as mosquitoes. Knowing the route allows control: covering the mouth and ventilation reduce droplet spread, proper cooking and clean water reduce food and water-borne disease, and controlling vectors such as removing stagnant water reduces malaria and dengue. Personal hygiene, safe food handling and public sanitation are key public-health measures. In data questions, identify the likely transmission route from the evidence and then suggest matching, specific control measures rather than generic advice.

Body defences

The body defends itself in layers. First-line, non-specific barriers stop pathogens entering: the skin acts as a physical barrier, mucus traps microbes in the airways, cilia sweep them away, and acid in the stomach and tears kill many microbes. If pathogens enter, white blood cells respond: phagocytes engulf and digest them by phagocytosis, while lymphocytes produce specific antibodies that bind to antigens on the pathogen, marking it for destruction or neutralising toxins. Blood clotting seals wounds and prevents entry. You should be able to distinguish non-specific defences, which act against any pathogen, from the specific immune response, which targets a particular pathogen and gives lasting protection.

Immunity and immunisation

Immunity is the ability to resist a particular disease. After an infection, some lymphocytes remain as memory cells, so a second exposure to the same pathogen produces a faster and larger antibody response that destroys it before symptoms appear; this is active immunity. Immunisation uses a vaccine containing weakened, dead or part of a pathogen to stimulate this memory without causing the disease, giving long-lasting protection and, when enough people are vaccinated, protecting the community. Passive immunity, such as antibodies received from the mother or by injection, gives immediate but short-lived protection because no memory cells form. You should be able to interpret antibody-level graphs comparing first and second responses and explain the difference.

Non-infectious disease

Non-infectious diseases are not caused by pathogens and cannot be passed from person to person. They include genetic disorders, deficiency diseases, degenerative diseases and cancer. Cancer arises when cell division becomes uncontrolled, forming a tumour; risk is raised by carcinogens such as the chemicals in tobacco smoke, ultraviolet radiation and certain dietary factors. Coronary heart disease results from fatty deposits narrowing the arteries that supply the heart, with risk increased by a high-fat diet, smoking, lack of exercise and stress. Many of these diseases are linked to lifestyle, so prevention focuses on diet, exercise and avoiding tobacco. You should be able to distinguish causes you can control from those, such as genetic factors, that you cannot.

Drugs and their effects

A drug is a substance that alters how the body works. Medicinal drugs treat disease, but misuse causes harm. Antibiotics kill or stop the growth of bacteria but have no effect on viruses, and their overuse encourages the evolution of resistant bacteria by natural selection, so they should be used only when needed and the full course completed. Recreational and abused drugs damage health: alcohol slows reactions and damages the liver, tobacco smoke harms the lungs and circulation, and many illegal drugs are addictive and damage the nervous system. You should be able to explain antibiotic resistance as a selection process and discuss the social as well as biological consequences of drug misuse in a balanced way.