Infection and response

1. Meet the pathogens

Some diseases can pass from one person to another, or between plants. These are called communicable diseases. They are caused by pathogens — tiny disease-causing organisms or viruses.

The four groups you need to recognise are bacteria, viruses, fungi and protists. They can infect animals or plants. The living thing they infect is called the host.

Not all microorganisms cause disease. Many bacteria are useful, including some that live in your gut.

2. How infections travel

Think about how a pathogen reaches its next host. Stopping that journey helps stop the disease spreading.

Isolation means keeping an infected person away from others while they could pass on the disease. Hygiene means actions such as washing hands and cleaning shared surfaces. Both reduce contact with pathogens.

3. Viral diseases: three examples

For each disease, ask four questions: What causes it? How does it spread? What does it do? How can we control it?

HIV is the virus. It attacks cells that normally defend the body. Without effective treatment, the damage can become so severe that the person struggles to fight other infections and some cancers. This late stage is called AIDS. HIV does not spread through everyday contact, such as sitting together in a classroom.

Why does TMV slow plant growth? Damaged leaves carry out less photosynthesis. They make less glucose, a sugar the plant uses for respiration and to build new plant material. Less glucose means less growth.

4. Bacterial diseases: food and sexual contact

Cross-contamination means pathogens moving from one food or surface to another. Imagine chopping raw chicken and then salad on the same unwashed board. Bacteria can move onto the salad, which will not be cooked. Cleaning the board or using separate boards helps prevent this.

5. Rose black spot and plant growth

Rose black spot is caused by a fungus. Dark purple or black spots appear on rose leaves. The leaves may turn yellow and fall off early.

The fungus produces tiny spores, which can grow into new fungi. Wind and splashes of water carry them to other leaves.

Yellow leaves absorb less light for photosynthesis. If leaves fall off, the plant also has less leaf area to absorb light. It makes less glucose and may grow more slowly.

Removing and destroying infected leaves removes a source of spores. A fungicide is a chemical that kills or stops the growth of fungi. It can help control black spot.

6. Malaria: the mosquito is the carrier

Malaria is caused by tiny single-celled organisms called protists. The disease can cause repeated bouts of fever and can be fatal.

A mosquito can pick up the protist when it feeds on an infected person’s blood. The protist spends part of its life cycle inside the mosquito. An infected female mosquito can then pass it to another person when it bites.

The mosquito is a vector: an animal that carries a pathogen between hosts. The protist causes malaria; the mosquito carries it.

• Mosquito nets reduce bites, so the protist has fewer chances to enter a person. Nets treated with insecticide can also kill mosquitoes.
• Removing suitable standing water reduces places where mosquitoes breed. Fewer mosquitoes means fewer possible carriers.

7. Your first line of defence

Your body has several ways to stop pathogens getting in. These are called non-specific defences because they work against many different pathogens.

Inside your airways, three parts work together:

• Goblet cells make mucus. These cells sit in the lining of the airways.
• Mucus is a sticky substance. It traps dust and pathogens before they can travel further into your lungs.
• Cilia move the mucus. These tiny hair-like structures beat towards the throat. The mucus can then be swallowed, and stomach acid kills many of the trapped pathogens.

If pathogens get past these barriers, white blood cells help defend your body.

8. White blood cells: three ways to defend you

Your immune system is your body’s defence against infection. Different white blood cells help in different ways.

1. Engulfing pathogens — phagocytosis

A white blood cell surrounds a pathogen and takes it inside. Enzymes then break it down. The scientific name for this is phagocytosis.

2. Making antibodies

Pathogens have molecules on their surface called antigens. White blood cells can recognise these and make antibodies — proteins that attach to them.

Each type of antibody has binding sites with a particular shape. These fit a matching antigen. Attaching antibodies helps the body identify and destroy the pathogen. An antibody that fits one antigen may not fit another.

3. Making antitoxins

Some bacteria release poisons called toxins. White blood cells can make antibodies called antitoxins. These attach to toxins and stop them causing harm. This is called neutralising the toxin.

9. Vaccination: prepare before infection

A vaccine prepares your immune system to recognise a pathogen before you catch the disease.

1. Show the body what to recognise. Some vaccines contain dead or inactive pathogens, or parts of them. Their antigens are recognised by white blood cells.
2. Make matching antibodies. The white blood cells respond by producing antibodies that fit those antigens. The vaccine does not cause the disease it protects against.
3. Remember the pathogen. Some white blood cells remain as memory cells. If the same pathogen enters later, the body can make the correct antibodies much faster and in larger amounts.

This faster response can destroy the pathogen before it makes you ill.

How does vaccination help other people? When many people are immune, fewer people catch and pass on the pathogen. It has fewer chances to reach someone who is not protected. This can help protect some unvaccinated people too, although it does not guarantee protection for everyone.

When judging a vaccination programme, consider how well it prevents illness, whether people can access it, its cost and how many people choose to take part.

10. Antibiotics, painkillers and resistance

Antibiotics are medicines that kill bacteria or stop them growing. Penicillin is one example. Different antibiotics work against different bacteria, so choosing the right one matters.

Antibiotics do not kill viruses. They target features of bacterial cells that viruses do not have. Viruses make copies of themselves inside our cells. A drug that stops this process could also harm our cells, which makes antiviral medicines harder to develop.

Painkillers reduce symptoms such as pain. They do not kill the pathogen. Feeling better does not always mean the infection has gone.

Why is antibiotic resistance a problem? Antibiotics have saved many lives, but some bacteria survive an antibiotic that used to kill them. These resistant bacteria can multiply and spread.

When an antibiotic kills other bacteria, resistant survivors have less competition. They reproduce, so more of the bacteria are resistant. Using antibiotics when they are not needed makes this problem worse.

11. Where medicines come from

Some important medicines were first discovered by studying plants and microorganisms.

Today, chemists make most new medicines using chemical reactions. This is called synthesis. A chemical found in a plant may still provide the starting idea.

A substance that kills bacteria in a dish is not automatically a safe medicine. Scientists must check what it does inside the body and how much can be given safely.

12. From a promising chemical to a tested medicine

Before a new medicine can be used, scientists need to answer three questions:

• Could it cause harm? This is testing its toxicity.
• Does it work? This is testing its efficacy.
• How much should be given? This is finding the right dose.

1. Test it before giving it to people. Scientists use cells and tissues in a laboratory, then live animals. These are called preclinical tests. They look for useful effects and possible harm.
2. Start trials with people carefully. Early trials usually involve healthy volunteers. Very low doses are given first to reduce the risk of harm.
3. Test it with patients. Scientists check whether it treats the illness. They also find the optimum dose: an amount that works well while keeping unwanted effects as low as possible.
4. Make a fair comparison. Patients are put into groups by chance. One group receives the new drug. Another receives a suitable comparison treatment, which may be a placebo — a treatment that looks the same but has no active drug. Sometimes an existing medicine is used for comparison instead.
5. Reduce bias. In a double-blind trial, neither the patients nor the people assessing the results know who received which treatment. Their expectations are then less likely to affect the results.
6. Ask other experts to check. Independent scientists check the methods and conclusions before the results are published. This is called peer review.

Choosing groups by chance is called random allocation. It helps avoid putting healthier patients into just one group. Testing more people makes the results less dependent on a few unusual cases. Even a well-run trial may miss rare or long-term side effects.

13. Read data and build an exam answer

Use these original exam-style questions to practise explaining your ideas. The optional challenges ask you to apply the same science with less help.

• Describe: say what happens or what you can see in the results.
• Explain: give a reason. Show how one thing causes another.
• Evaluate: use the evidence to decide how strong a claim is. Include what the evidence cannot tell you.

Before reading a graph, check what each axis shows and the units. Use numbers from the graph to support your answer. A difference between two groups does not, on its own, prove what caused it.

Writing a six-mark answer: plan the main steps, then link them together. Explain how and why things happen. Try the question before opening the example answer.

Use a graph to explain protection

Read the graph, compare the two groups and use the evidence to explain how vaccination helps.

Your answer sheet

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One final thought

Use a scientific idea and a specific example in your reflection.