What you'll learn
Infection and response is the unit of AQA GCSE Combined Science: Trilogy that explains communicable disease: what causes it, how it spreads, how the body fights it, and how medicine helps. Communicable diseases are caused by pathogens, which are microorganisms that cause infectious disease, and they fall into four groups — bacteria, viruses, protists and fungi. By the end of this unit you should be able to name a named example of a disease from each group together with its symptoms, transmission and prevention; describe the body's non-specific defences and the three actions of white blood cells; explain how vaccination produces immunity and why it works before the person is ever exposed; distinguish antibiotics from painkillers and explain why antibiotic resistance is a growing problem; and describe how new drugs are discovered and tested. This unit appears on Biology Paper 1 and rewards precise use of vocabulary more than almost any other.
Key terms and definitions
Pathogen — a microorganism that causes infectious disease
Communicable disease — a disease that can be passed from one organism to another
Antigen — a unique protein on the surface of a pathogen that the immune system recognises
Antibody — a protein produced by white blood cells that binds to a specific antigen and causes pathogens to clump together
Antitoxin — a protein produced by white blood cells that neutralises the toxins released by pathogens
Vaccination — introducing small quantities of dead or inactive forms of a pathogen to stimulate white blood cells to produce antibodies
Herd immunity — the protection given to unvaccinated individuals when a large proportion of the population is immune, so the pathogen cannot spread easily
Antibiotic — a medicine that kills bacteria inside the body without harming body cells
Antibiotic resistance — the ability of a strain of bacteria to survive treatment with an antibiotic that would previously have killed it
Placebo — a substance with no active drug in it, given in a clinical trial so that the effect of the drug itself can be measured
Double blind trial — a trial in which neither the patient nor the doctor knows who has received the drug and who has received the placebo
Core concepts
How pathogens spread
Pathogens reproduce rapidly inside the body. Bacteria do damage by producing toxins that harm tissues and make us feel ill. Viruses do damage differently: they enter a cell, use it to make many copies of themselves, and then burst the cell open, which is what causes the symptoms.
There are three routes of transmission you need. In the air, pathogens travel in droplets released when an infected person coughs or sneezes. By direct contact, they pass through touch, including contact with contaminated surfaces or through sexual contact. By water or food, they are swallowed in contaminated material. Every prevention measure you can name works by blocking one of these routes — covering the mouth when sneezing, washing hands, treating water, and isolating infected individuals.
Named bacterial diseases
Salmonella food poisoning is caused by bacteria ingested in food that has been prepared in unhygienic conditions or in poultry that was infected. The bacteria secrete toxins, causing fever, abdominal cramps, vomiting and diarrhoea. In the UK, poultry are vaccinated against salmonella to control it.
Gonorrhoea is a sexually transmitted disease. Its symptoms are a thick yellow or green discharge and pain on urination. It was once treated easily with penicillin, but resistant strains have appeared, so other antibiotics are now used. It is prevented by using a barrier method of contraception such as a condom, and by treating infected individuals.
Named viral diseases
Measles is spread by inhaling droplets from sneezes and coughs. It causes fever and a red skin rash and can be fatal if complications arise, so most young children are vaccinated against it.
HIV initially causes flu-like illness. It is spread by sexual contact or by exchange of body fluids such as blood, which is why sharing needles transmits it. Unless it is controlled with antiretroviral drugs, the virus attacks the body's immune cells. Late-stage HIV infection, or AIDS, occurs when the immune system is so badly damaged that it can no longer deal with other infections or cancers.
Tobacco mosaic virus is a plant pathogen affecting many species including tomatoes. It produces a distinctive mosaic pattern of discolouration on the leaves, and because the affected areas contain less chlorophyll, the rate of photosynthesis falls and growth is stunted.
Named fungal and protist diseases
Rose black spot is a fungal disease in which purple or black spots develop on the leaves of roses. The leaves then turn yellow and drop early, reducing photosynthesis and slowing growth. It spreads in the environment by water and wind, and is treated with fungicides or by removing and destroying the affected leaves.
Malaria is caused by a protist. Mosquitoes act as vectors, carrying the protist from person to person when they feed on blood. Malaria causes recurrent episodes of fever and can be fatal. Control depends on preventing the vector from breeding and on stopping mosquitoes from biting people, for example by using insecticide-treated nets.
Human defence systems
Before the immune system is involved at all, the body has non-specific defences that simply keep pathogens out. The skin forms a barrier and produces antimicrobial secretions, and scabs seal wounds. The nose has hairs and mucus that trap particles. The trachea and bronchi are lined with mucus that traps pathogens and with cilia that waft the mucus up to the throat to be swallowed. The stomach produces hydrochloric acid, which kills most pathogens that are swallowed.
If a pathogen gets past these, white blood cells act in three ways. Phagocytosis is the engulfing and digesting of pathogens. Antibody production targets the specific antigens on a particular pathogen, causing the pathogens to clump together so they are destroyed more easily. Antitoxin production neutralises the toxins that bacteria release.
Vaccination and immunity
A vaccine contains small quantities of a dead or inactive form of a pathogen. Because the antigens are still present, white blood cells respond by producing the specific antibodies against them, but the person does not become ill. Crucially, the body retains memory cells. If the live pathogen later enters the body, antibodies are produced far more quickly and in much greater quantity, so the pathogen is destroyed before it can cause disease. The person is immune.
If a large proportion of a population is vaccinated, the spread of a pathogen is greatly reduced because there are too few susceptible people for it to move between. This herd immunity protects those who cannot be vaccinated, such as very young babies or people with weakened immune systems.
Antibiotics, painkillers and resistance
Antibiotics such as penicillin kill bacteria inside the body. They are specific, so the correct antibiotic must be used for a particular bacterial infection. Antibiotics cannot kill viruses, because viruses live and reproduce inside the body's own cells, so a drug able to reach them would also damage those cells. This is why developing drugs against viral pathogens is difficult.
Painkillers and similar medicines treat the symptoms of disease but do not kill pathogens at all. Confusing the two loses marks routinely.
Antibiotic resistance arises by natural selection. Random mutation produces a bacterium that survives the antibiotic. When the antibiotic is used, the non-resistant bacteria die and the resistant one survives and reproduces, passing the resistance gene on, so a resistant strain spreads. Overuse and inappropriate use of antibiotics speed this up, as does failing to complete the full course, which leaves partially resistant survivors. Doctors are therefore asked not to prescribe antibiotics for non-serious or viral infections, and patients are asked to complete every course.
Discovering and testing new drugs
Traditionally, drugs were extracted from plants and microorganisms. Digitalis, a heart drug, originates from foxgloves; aspirin, a painkiller, originates from willow; and penicillin was discovered by Alexander Fleming from the Penicillium mould. Most new drugs today are synthesised by chemists, though the starting point may still be a natural compound.
New drugs are tested for toxicity, efficacy and dose. Preclinical testing is carried out in a laboratory using cells, tissues and live animals. Drugs that pass move to clinical trials on healthy volunteers, starting at a very low dose to check for safety, and then on patients to find the optimum dose. Trials are usually double blind, with some patients given a placebo, and the results are peer reviewed before publication to prevent false claims.
Worked examples
Example 1: Explaining how a vaccine works (4 marks)
Explain how being vaccinated against measles protects a child from the disease.
The vaccine contains a dead or inactive form of the measles virus carrying its antigens. White blood cells recognise these antigens and produce specific antibodies against them, and memory cells remain in the body afterwards. If the child later meets the live measles virus, antibodies are produced much more rapidly and in much greater quantity, so the virus is destroyed before it can make the child ill.
Example 2: Explaining antibiotic resistance (4 marks)
Explain how a strain of bacteria resistant to an antibiotic can develop and spread.
A random mutation in a bacterium produces resistance to the antibiotic. When the antibiotic is used, non-resistant bacteria are killed but the resistant bacterium survives. It reproduces, passing on the gene for resistance, so the proportion of resistant bacteria in the population increases. Over time the resistant strain becomes common and the antibiotic no longer works, and the strain can then spread between people because there is no effective treatment.
Example 3: Interpreting a trial result (3 marks)
In a clinical trial, 40 per cent of patients given a new drug improved, compared with 35 per cent of patients given a placebo. Suggest what the researchers should conclude.
The improvement in the drug group is only five percentage points greater than in the placebo group, which is a small difference and may be due to chance rather than to the drug. The researchers should conclude that the trial has not shown the drug to be effective, and should repeat the trial with a larger sample before drawing any firm conclusion.
Common mistakes and how to avoid them
The most damaging error in this unit is writing that antibiotics kill viruses. They do not, and the sentence usually costs more than one mark because the following reasoning collapses with it.
Students frequently confuse antigens with antibodies. The antigen is on the pathogen; the antibody is made by the white blood cell. A useful memory aid is that antibodies are anti, meaning against, the antigens.
Another frequent slip is saying that a vaccine gives you the disease in a mild form. The specification's wording is a dead or inactive form of the pathogen, and that is what should be written.
In resistance questions, many students write that bacteria become resistant because of the antibiotic. Mutation is random and happens first; the antibiotic then selects for the bacteria that already have it. Getting this order the wrong way round is the single biggest source of lost marks in natural selection answers.
Finally, when naming a disease, give the pathogen type as well. Writing that malaria is caused by mosquitoes is wrong: mosquitoes are the vector, and the pathogen is a protist.
Exam technique for "Biology: Infection and Response"
Learn the named diseases as a table in your head: disease, type of pathogen, symptoms, transmission, prevention or treatment. Questions often ask for two of those five, and having the full set ready means you are never caught out by which two.
Where a question asks you to explain rather than describe, it wants mechanism. Describing that white blood cells fight infection is a description; explaining that they engulf and digest the pathogen, or produce antibodies specific to its antigens, is an explanation.
Data questions about vaccination rates and disease incidence are common. Quote figures from the data with their units, and if asked to evaluate, mention sample size, the time period covered, and whether other factors could explain the trend.
For drug testing, remember the order — preclinical on cells and tissues, then animals, then healthy volunteers, then patients — because questions often give a stage and ask what comes next and why.
Quick revision summary
Pathogens are bacteria, viruses, protists and fungi, spread through air, direct contact, or water and food. Bacteria produce toxins; viruses reproduce inside cells and burst them. Named examples are salmonella and gonorrhoea, measles, HIV and tobacco mosaic virus, rose black spot, and malaria carried by a mosquito vector. Non-specific defences are the skin, nose, trachea and bronchi with mucus and cilia, and stomach acid. White blood cells carry out phagocytosis and produce antibodies and antitoxins. Vaccines contain dead or inactive pathogen, stimulate antibody production and leave memory cells, giving a faster and larger response on later exposure. Antibiotics kill bacteria but not viruses; painkillers treat symptoms only. Resistance arises by random mutation followed by natural selection. New drugs are tested preclinically and then in double blind clinical trials against a placebo.