Section 1 of 9Pathogens
A pathogen is a microorganism that causes disease. Four types are important at GCSE.
Bacteria
Bacteria are single-celled organisms without a nucleus that reproduce rapidly. They damage tissues and release toxins, for example Salmonella causing food poisoning, and tuberculosis and gonorrhoea.
Viruses
Viruses are tiny particles that live and reproduce inside host cells, damaging them. Examples: measles, HIV, tobacco mosaic virus in plants.
Fungi
Fungi can grow in the body or on plants, for example athlete's foot and rose black spot.
Protists
Protists are single-celled eukaryotes, some of which are parasites, such as the malaria parasite Plasmodium.
Section 2 of 9How pathogens spread
| Route | Example | How it is controlled |
|---|---|---|
| Air (droplets, coughs and sneezes) | Measles, flu, tuberculosis | Vaccination, tissues and covering the mouth, isolation |
| Direct contact | Athlete's foot, HIV (blood, sexual contact) | Hygiene, condoms, not sharing needles, antifungal treatment |
| Water and food | Cholera, Salmonella | Clean water, sewage treatment, cooking food thoroughly, hygiene in kitchens |
| Vectors (animals that carry the pathogen) | Malaria (mosquito) | Mosquito nets, insecticide, draining breeding sites, drugs |
Reducing the spread of disease includes personal hygiene, washing hands, destroying vectors, isolating infected people, and vaccination.
Section 3 of 9Named diseases
Measles
(virus): spread by droplets; causes fever and a red rash, and can lead to pneumonia, brain damage or death. Prevented by the MMR vaccine.
HIV
(virus): spread by sexual contact, blood and from mother to baby. It infects white blood cells, causing the immune system to fail, which leads to AIDS. Antiretroviral drugs control the virus but do not cure it.
Tobacco mosaic virus
Tobacco mosaic virus infects plants such as tobacco and tomato, causing a mosaic pattern of discoloured leaves; chloroplast damage reduces photosynthesis and growth.
Salmonella
(bacteria): food poisoning, causing fever, vomiting and diarrhoea, from undercooked poultry or eggs. Poultry are vaccinated in the UK.
Gonorrhoea
(bacteria): a sexually transmitted infection producing pain and a thick discharge; it is treated with antibiotics but resistant strains have appeared. Barrier methods like condoms prevent spread.
Rose black spot
(fungus): purple-black spots on leaves, which then turn yellow and fall off; photosynthesis is reduced. It spreads by spores in water and wind and is controlled by fungicides and by removing and burning infected leaves.
Malaria
(protist): spread by the female Anopheles mosquito, which takes in the parasite when it feeds on infected blood.
- It causes repeated fevers and can damage the liver and blood.
- It is controlled by preventing mosquito bites rather than by killing the protist in the environment.
Section 4 of 9Why are some diseases hard to control?
Factors include
- pathogens that mutate and change (flu, HIV), so that vaccines or the immune memory do not work;
- resistance to antibiotics;
- high population density and travel, which move infection quickly;
- poverty and lack of healthcare, clean water and sanitation;
- and people who carry a pathogen without symptoms.
Why are some diseases hard to control?
Epidemics can spread rapidly in countries where vaccination levels are low.
Health services track outbreaks, trace contacts and use quarantine to limit the spread.
Section 5 of 9Spread by droplets, contact and vectors: more detail
Pathogens leave the body by routes that depend on where they live.
1
Those in the lungs and throat leave in droplets when an infected person coughs, sneezes or talks; each sneeze can contain thousands of tiny droplets that can be inhaled by others.
2
Gut pathogens such as Salmonella and the cholera bacterium leave in faeces or vomit and spread when water or food is contaminated.
3
Cholera produces a toxin that causes severe diarrhoea; the loss of water and salts can kill if not treated by rehydration, so it is most dangerous where sewage contaminates drinking water.
4
HIV is found in body fluids and passes on during unprotected sex, by sharing needles or from mother to baby.
5
The virus is not spread by touch, coughing or sharing cups.
The idea of a reservoir and transmission route helps in planning control: for each disease decide where the pathogen lives, how it moves, and where the chain can be broken. For malaria the chain can be broken by treating patients, by killing vectors or by stopping mosquitoes biting; for measles the best break is vaccination, which protects people before exposure.
Section 6 of 9Treatment and the limits of antibiotics
Antibiotics such as penicillin kill bacteria or stop their growth by damaging the cell wall or processes in the bacterial cell without harming human cells.
Treatment and the limits of antibiotics
They are useless against viruses, because viruses live inside our cells and have no cell structures for the drugs to target; antiviral drugs interfere with virus reproduction but are specific to each virus.
Antifungal creams treat athlete's foot.
Bacterial resistance to antibiotics is increasing because of overuse, which leads to selection of resistant strains, so antibiotics should be used only when needed and the full course completed.
Section 7 of 9Exam-style questions
Describe how malaria is spread and suggest two ways of reducing its spread. [4]4 marks
11. The malaria protist is carried by mosquitoes (vector). [1]22. A mosquito picks up the parasite when it bites an infected person and passes it on when it bites another. [1]33. Reduce spread by using mosquito nets or insect repellent. [1]44. Drain standing water or use insecticides to reduce breeding. [1]Explain why washing hands thoroughly after using the toilet helps to control cholera and Salmonella. [2]2 marks
1. Pathogens in faeces can be transferred to food or water by hands. [1]
2. Washing removes them, so other people do not ingest them. [1]
Tobacco mosaic virus causes leaves to have discoloured patches. Explain why infected plants grow poorly. [3]3 marks
Exam-style questions1. The discoloured patches have less chlorophyll. [1]2. So less light is absorbed and photosynthesis is reduced. [1]3. Less glucose is made for growth and respiration. [1]
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Pathogen | A microorganism that causes disease. |
| Vector | An organism that carries a pathogen from one host to another. |
| Virus | A tiny non-cellular particle that reproduces inside host cells. |
| Bacterium | A single-celled organism without a nucleus. |
| Protist | A single-celled eukaryote, some of which are parasites. |
| Fungus | An organism that may cause disease by growing on or in a host. |
| Epidemic | A rapid spread of a disease through a population. |
| Antibiotic | A drug that kills bacteria. |
Section 9 of 9Exam technique
1
Saying that antibiotics can treat viral diseases like measles or HIV. They only kill bacteria.
2
Writing that malaria is spread by the mosquito 'being infected by a virus'. The pathogen is a protist.
3
Thinking that vaccines cure existing infections; they prevent them.
4
Giving 'cleaning' as a control method without saying what is removed and how it breaks the spread.
5
Mixing up the vector (mosquito) and the pathogen (Plasmodium).