Health, communicable & non-communicable disease

Health is a state of complete physical and mental wellbeing, not just the absence of disease. Diseases fall into two groups:

• Communicable (infectious) diseases — caused by pathogens and can spread between organisms (e.g. flu, cholera, malaria).
• Non-communicable diseases — cannot be passed on; they generally last a long time and get worse slowly (e.g. cardiovascular disease, type 2 diabetes, many cancers).

Diseases can interact: having one disease can make another more likely — e.g. a virus can trigger some cancers, and a weakened immune system (as in HIV/AIDS) makes other infections more likely.

The four types of pathogen

Pathogens are microorganisms that cause communicable disease. There are four types — learn a named example of each:

• Bacteria — e.g. cholera (diarrhoea, spread by dirty water) and tuberculosis (TB) (lungs, spread by droplets).
• Viruses — e.g. HIV (attacks the immune system) and the flu virus.
• Fungi — e.g. Chalara ash dieback, which kills ash trees.
• Protists — e.g. the malaria parasite, spread by mosquito vectors.

How bacteria and viruses harm us: bacteria release toxins that damage cells; viruses reproduce inside cells and burst them, damaging tissue.

Name that pathogen

Match disease to pathogen type

Tap a disease, then tap the type of pathogen that causes it.

Sexually transmitted infections

Some pathogens are spread through sexual contact — these cause sexually transmitted infections (STIs):

• Chlamydia — caused by a bacterium; a common STI that can cause infertility. Often has no symptoms.
• HIV — caused by a virus; attacks the immune system and can lead to AIDS. Also spread by blood (e.g. shared needles).

Prevention: using condoms, screening and treating infected people, avoiding sharing needles, and reducing the number of sexual partners all reduce the spread of STIs.

Plant defences

Plants defend themselves against pathogens and pests using physical and chemical barriers:

• Physical: a waxy cuticle and cellulose cell walls stop pathogens entering; layers of dead cells (bark) fall off carrying pathogens with them.
• Chemical: some plants make antibacterial chemicals or poisons to deter herbivores (e.g. in mint and witch hazel).

Uses to humans: chemicals plants produce for defence can be used to make medicines and pest-control products.

Human defences & the immune system

The body first tries to stop pathogens entering with barriers, then the immune system destroys any that get in.

• Physical/chemical barriers: skin; mucus and cilia in the airways; stomach acid (kills swallowed pathogens); lysozyme in tears.
• Immune response: white blood cells called lymphocytes recognise the pathogen's antigens and make specific antibodies that destroy it.
• After an infection, memory cells remain. If the same pathogen returns they make antibodies faster and in greater numbers, so you are immune.

Why you don't catch it twice

Vaccination & herd immunity

A vaccine contains a dead or weakened (inactive) form of a pathogen, or its antigens. It cannot make you ill, but:

• Your lymphocytes respond to the antigens by making antibodies.
• Memory cells are produced, so if the real pathogen infects you later, the immune response is fast enough to destroy it before you get ill.

Herd immunity: when a large proportion of the population is vaccinated, the pathogen cannot spread easily, so even unvaccinated people are protected.

Evaluating vaccination: it has hugely reduced diseases such as measles and polio. Very rarely there can be side effects, and no vaccine is 100% effective — but the benefits to the population are large.

Antibiotics & resistance

Antibiotics (e.g. penicillin) kill bacteria inside the body, so they cure bacterial diseases. Important points:

• Antibiotics do not kill viruses, so they don't work on colds or flu.
• Antibiotic resistance evolves by natural selection: a random mutation makes some bacteria resistant, they survive treatment and reproduce, so resistant strains (e.g. MRSA) spread.

Reducing resistance: only use antibiotics when necessary, don't use them for viral infections, and always finish the whole course so no surviving bacteria remain.

Developing new medicines

New drugs are tested in stages to check they are safe, effective, the right dose, and stable:

• Preclinical testing — on cells, tissues and then animals in the lab.
• Clinical trials — on healthy volunteers first (safety and dose), then on patients (does it work?).
• A placebo (a dummy with no drug) is used for comparison.
• Trials are often double-blind: neither the patient nor the doctor knows who gets the real drug, to prevent bias.

Peer review: before results are trusted, other scientists check them. This prevents false claims and errors.

Why use a placebo?

Risk factors, BMI & waist:hip ratio

Non-communicable diseases are linked to risk factors such as smoking, a poor diet, lack of exercise, alcohol and obesity. Two measures used to assess obesity:

Interpreting BMI: roughly, under 18.5 = underweight, 18.5–24.9 = healthy, 25–29.9 = overweight, 30+ = obese. A high waist:hip ratio also indicates a higher risk of cardiovascular disease.

Your turn — calculate BMI

Your turn — waist:hip ratio

Cardiovascular disease & treatments

Cardiovascular disease (CVD) is a non-communicable disease of the heart and blood vessels. Fatty deposits can narrow the coronary arteries, reducing blood flow to the heart muscle. Treatments include:

• Stents — wire mesh tubes that hold a narrowed artery open, restoring blood flow. Fast-acting, but surgery has risks.
• Statins — drugs taken long-term that lower ‘bad’ (LDL) cholesterol in the blood, slowing fatty deposits. Can have side effects.
• Lifestyle changes — healthy diet, exercise and not smoking reduce the risk in the first place.

Evaluate treatments: weigh benefits (e.g. stents give fast relief) against risks and costs (surgery risks; statins must be taken for life). Lifestyle change is cheapest and prevents CVD.

Your turn — BMI again

Match the term to its role

Tap a term on the left, then its role on the right.

The big ideas to know

You've covered the core of Edexcel Topic 5 — Health, disease and the development of medicines. Press Finish to see your score.