Communicable Diseases and Pathogens Notes
AQA GCSE Biology: Revision notes
Key facts
- A pathogen is a microorganism that causes disease: bacteria, viruses, fungi or protists.
- Pathogens cause disease by toxins (from outside cells) or by directly damaging cells (viruses reproduce inside them).
- Spread is by direct contact, water or food, air or vectors.
- Match the disease to its route: measles and TB (air), Salmonella (food), gonorrhoea (contact), malaria (mosquito).
- Prevention targets the route: isolation, hygiene, vaccination, vector control, antibiotics.
Types of pathogen
Four types of microorganism cause disease: bacteria, viruses, fungi and protists.
A pathogen is a microorganism that causes disease in a host. Each type differs in structure and reproduction.
Examiners expect you to tell living pathogens (bacteria, fungi, protists) from non-living ones. Viruses are not cells and can only reproduce inside host cells.
Bacteria
- Single-celled prokaryotes
- Binary fission
- Tuberculosis, Salmonella
Viruses
- Particles, not cells
- Reproduce inside host cells
- Measles, HIV, tobacco mosaic virus
Fungi
- Eukaryotes: yeasts or moulds
- Reproduce by spores
- Rose black spot
Protists
- Single-celled eukaryotes
- Larger than bacteria
- Malaria (Plasmodium)
Which type of pathogen is non-living and can only reproduce inside host cells?
How pathogens cause disease
Pathogens release toxins or damage cells directly; bacteria can act from outside cells, viruses from within.
Toxins: many bacteria release toxins, poisonous substances that damage cells and tissues without the pathogen entering cells. Cholera toxins damage the intestinal lining, causing severe diarrhoea.
Direct damage: viruses reproduce inside host cells and burst them open, causing cell death. Fungi grow through tissues, breaking down cells.
This cell damage stops normal function and causes the symptoms.
- 1
Bacteria release toxins
Poisons damage cells from outside, e.g. cholera in the gut
- 2
Viruses enter cells
They reproduce inside the host cell
- 3
Cells burst
Host cells die as new viruses escape
- 4
Symptoms
Damaged cells cannot work normally
Toxins
- Poisonous substances released by many bacteria
- Damage cells from outside
- Example: cholera
Direct damage
- Pathogen invades and destroys cells
- Viruses burst cells open as they reproduce
- Fungi break down cell walls and membranes
How do bacteria such as cholera bacteria cause damage?
How diseases spread
Communicable diseases spread between organisms by direct contact, water or food, air or vectors.
Communicable diseases spread from one organism to another.
- Direct contact: touching infected skin or body fluids, such as gonorrhoea.
- Water or food: contaminated drinking water or food, such as Salmonella.
- Air: droplets from coughs and sneezes, such as measles and tuberculosis.
- Vectors: animals that carry pathogens without harm, such as mosquitoes transmitting malaria.
- 1
Direct contact
Skin or body fluids, e.g. gonorrhoea
- 2
Water or food
Contaminated, e.g. Salmonella
- 3
Air
Droplets, e.g. measles and TB
- 4
Vectors
Carrier animals, e.g. mosquitoes and malaria
| Direct contact | Water or food | Air | Vectors | |
|---|---|---|---|---|
| How | Touching infected skin or body fluids | Contaminated drink or food | Droplets from coughing and sneezing | Animals that carry pathogens |
| Example | Gonorrhoea | Salmonella | Measles, tuberculosis | Malaria (mosquito) |
Direct contact
- How:
- Touching infected skin or body fluids
- Example:
- Gonorrhoea
Water or food
- How:
- Contaminated drink or food
- Example:
- Salmonella
Air
- How:
- Droplets from coughing and sneezing
- Example:
- Measles, tuberculosis
Vectors
- How:
- Animals that carry pathogens
- Example:
- Malaria (mosquito)
Which disease is spread by a vector?
Specific diseases
Learn the pathogen, route and key symptoms for each disease.
Each pathogen causes characteristic symptoms depending on the organs it affects. Use correct names: Mycobacterium tuberculosis, not just 'the TB bacterium'.
HIV attacks the immune system and can progress to AIDS, but antiretroviral drugs control it. Salmonella is controlled by vaccinating poultry. Rose black spot is treated with fungicides and removing infected leaves.
| Tuberculosis | Salmonella | Gonorrhoea | |
|---|---|---|---|
| Pathogen | Mycobacterium tuberculosis | Salmonella bacterium | Neisseria gonorrhoeae |
| Spread | Air (coughing droplets) | Contaminated food or water | Direct contact (sexual) |
| Symptoms | Persistent cough, chest pain, fever, weight loss | Diarrhoea, cramps, vomiting | Discharge, painful urination; may have no symptoms |
Tuberculosis
- Pathogen:
- Mycobacterium tuberculosis
- Spread:
- Air (coughing droplets)
- Symptoms:
- Persistent cough, chest pain, fever, weight loss
Salmonella
- Pathogen:
- Salmonella bacterium
- Spread:
- Contaminated food or water
- Symptoms:
- Diarrhoea, cramps, vomiting
Gonorrhoea
- Pathogen:
- Neisseria gonorrhoeae
- Spread:
- Direct contact (sexual)
- Symptoms:
- Discharge, painful urination; may have no symptoms
| Measles | HIV | Tobacco mosaic virus | |
|---|---|---|---|
| Pathogen | Measles virus | Human immunodeficiency virus | TMV |
| Spread | Air | Blood and body fluids | Contact with infected plants and sap |
| Symptoms | Rash, fever, cough | Flu-like at first, then AIDS | Mottled leaves, reduced growth |
Measles
- Pathogen:
- Measles virus
- Spread:
- Air
- Symptoms:
- Rash, fever, cough
HIV
- Pathogen:
- Human immunodeficiency virus
- Spread:
- Blood and body fluids
- Symptoms:
- Flu-like at first, then AIDS
Tobacco mosaic virus
- Pathogen:
- TMV
- Spread:
- Contact with infected plants and sap
- Symptoms:
- Mottled leaves, reduced growth
| Rose black spot | Malaria | |
|---|---|---|
| Pathogen | Fungus | Plasmodium (protist) |
| Spread | Spores spread by water and air | Mosquito bites |
| Symptoms | Black or brown spots, leaf drop | Fever, chills, headache, recurring cycles |
Rose black spot
- Pathogen:
- Fungus
- Spread:
- Spores spread by water and air
- Symptoms:
- Black or brown spots, leaf drop
Malaria
- Pathogen:
- Plasmodium (protist)
- Spread:
- Mosquito bites
- Symptoms:
- Fever, chills, headache, recurring cycles
Which pathogen type causes malaria?
Preventing spread
Prevention targets the route of spread, and a combination of methods is usually needed.
Match prevention to the transmission route.
- Air: isolate infected people, vaccinate and cover coughs and sneezes.
- Water or food: clean water supply, sewage treatment and food hygiene.
- Direct contact: barriers such as condoms and safe practices.
- Vectors: insecticides, bed nets, draining standing water.
Antibiotics treat bacterial infections early. For plants, remove infected plants and use fungicides.
- 1
Find the route
Air, water or food, contact or vector
- 2
Block that route
Isolation, hygiene, barriers or insecticides
- 3
Protect hosts
Vaccinate
- 4
Treat early
Antibiotics for bacterial infections
Air
- Isolation
- Vaccination
- Cover coughs
Water or food
- Clean water
- Sewage treatment
- Food hygiene
Direct contact
- Barrier methods
- Safe sexual practices
Vectors
- Insecticides
- Bed nets
- Drain standing water
Which method best prevents the spread of Salmonella?
Try an exam question
Malaria is a communicable disease. Name the type of pathogen that causes it and how it is spread. Suggest two ways to reduce the spread of malaria.
[5 marks]
- [1]The pathogen is a protist (Plasmodium).
- [1]It is spread by a vector, the mosquito, which bites infected and uninfected people.
- [1]Use insecticides to kill mosquitoes.
- [1]Use bed nets to prevent bites.
- [1]Drain standing water (or use repellents) so mosquitoes cannot breed (any two reductions).
That's the notes covered.
Carry on to the next subtopic.