Infection and Response — AQA Combined Science: Trilogy

This unit is about pathogens (disease-causing microorganisms), how they spread, how the body defends itself, and how we prevent and treat disease with vaccines and medicines.

Pathogens and communicable disease

Pathogens are microorganisms that cause communicable (infectious) diseases. They can be spread between organisms. The four types are bacteria, viruses, fungi and protists.

• Bacteria reproduce rapidly and can make toxins that damage tissues.
• Viruses live and reproduce inside cells, bursting them and causing damage.

How pathogens spread

• Direct contact (e.g. some skin and sexually transmitted infections).
• Water (e.g. drinking contaminated water).
• Air (droplets when sneezing or coughing).

Spread can be reduced by hygiene (hand washing, disinfectant), isolating infected individuals, destroying vectors, and vaccination.

Examples of diseases (learn these)

Viral:

• Measles — spread by droplets; fever and rash; can be fatal. Prevented by vaccination (MMR).
• HIV — spread by bodily fluids/sharing needles; attacks immune cells; controlled with antiretroviral drugs. Late stage is AIDS.
• Tobacco mosaic virus (TMV) — a plant disease that gives leaves a mosaic pattern, reducing photosynthesis.

Bacterial:

• Salmonella — food poisoning from contaminated food; fever, cramps, vomiting, diarrhoea. Poultry are vaccinated to control it.
• Gonorrhoea — sexually transmitted; treated with antibiotics (though resistant strains are increasing); spread reduced by barrier contraception.

Fungal:

• Rose black spot — purple/black spots on leaves, reducing photosynthesis; spread in water/wind; treated with fungicides.

Protist:

• Malaria — caused by a protist spread by mosquito vectors; causes recurrent fevers; controlled by preventing mosquito breeding and using mosquito nets.

Human defence systems

The body's non-specific first lines of defence stop pathogens entering:

• Skin — a barrier; also produces antimicrobial secretions.
• Nose — hairs and mucus trap particles.
• Trachea and bronchi — mucus traps pathogens; cilia waft it away.
• Stomach — hydrochloric acid kills pathogens in food.

If pathogens get inside, white blood cells defend the body by:

1. Phagocytosis — engulfing and digesting pathogens.
2. Producing antibodies — proteins that lock onto specific antigens on pathogens, marking them for destruction.
3. Producing antitoxins — that neutralise toxins released by bacteria.

Vaccination

A vaccine contains small quantities of dead or inactive forms of a pathogen (or its antigens). This stimulates white blood cells to produce antibodies. If the real pathogen later enters, the body responds quickly and in large numbers to destroy it, so the person does not become ill.

Herd immunity: if a large proportion of the population is vaccinated, the spread of a pathogen is greatly reduced, protecting even those who are not immune.

You should be able to evaluate vaccination programmes, weighing benefits (preventing epidemics) against possible drawbacks (rare side effects, cost).

Medicines

• Painkillers (e.g. aspirin, paracetamol) treat symptoms but do not kill pathogens.
• Antibiotics (e.g. penicillin) kill bacteria inside the body and have greatly reduced deaths from bacterial diseases.

Important points:

• Antibiotics do not kill viruses (viruses live inside cells, making them hard to target without harming the body).
• Antibiotic-resistant bacteria (e.g. MRSA) are increasing because of overuse and not completing courses. Resistance is reduced by not over-prescribing antibiotics and always finishing the course.

Origins of drugs

Many drugs were originally extracted from plants and microorganisms:

• Digitalis (heart drug) from foxgloves.
• Aspirin from willow.
• Penicillin discovered by Alexander Fleming from the Penicillium mould.

Most new drugs are now synthesised by chemists, but the starting point may still be a natural compound.

Developing new drugs

New drugs are extensively tested for safety, efficacy (does it work), toxicity and dose before use:

1. Preclinical testing — on cells, tissues and live animals.
2. Clinical trials — on healthy volunteers (low doses, to check safety) then on patients to find the optimum dose and check it works.
3. Double-blind trials — some patients get a placebo; neither doctors nor patients know who receives which, to remove bias.

Results are peer reviewed and published. You should be able to explain why each stage is important.

Exam tips

• Learn the named diseases with their type, symptoms, spread and prevention — these come up every year.
• Be clear that vaccines work before infection (preparing the immune system), while antibiotics treat existing bacterial infections.
• Explain antibiotic resistance using natural selection: resistant bacteria survive and reproduce.
• Remember the purpose of placebos and double-blind trials is to remove bias.