Communicable Diseases and PathogensAQA GCSE Biology: Revision notes
Section 1
What is a pathogen and what are the main types?
A pathogen is a microorganism that causes disease in a host organism. There are four main types of pathogens:
| Pathogen Type | Characteristics | Examples |
|---|---|---|
| Bacteria | Single-celled prokaryotes; reproduce by binary fission; typically 1–5 μm | Tuberculosis, Salmonella |
| Viruses | Non-living particles; contain genetic material (DNA or RNA) surrounded by protein coat; reproduce inside host cells | Measles, HIV, tobacco mosaic virus |
| Fungi | Eukaryotic organisms; some are single-celled (yeasts), others multicellular (moulds); reproduce via spores | Rose black spot |
| Protists | Single-celled eukaryotes; larger than bacteria; often found in aquatic environments | Malaria parasite (Plasmodium) |
Each type has different structures and reproduction methods, which affects how they cause disease and how they spread.
Examiners expect you to distinguish between living pathogens (bacteria, fungi, protists) and non-living ones (viruses). Remember that viruses are not cells and require host cells to reproduce.
Section 2
How do pathogens cause disease?
Pathogens cause disease through two main mechanisms:
1. Producing toxins
- Many bacteria release toxins (poisonous substances) into the host's body
- These toxins damage host cells and tissues without the pathogen necessarily invading cells directly
- Example: Cholera bacteria produce toxins that damage the intestinal lining, causing severe diarrhoea
2. Directly damaging cells
- Pathogens can invade and destroy host cells during replication
- Viruses reproduce inside host cells, bursting them open and causing cell death
- Fungi grow through tissues, breaking down cell walls and membranes
- This cell damage prevents normal function and causes symptoms
The type of damage depends on the pathogen's structure and reproduction strategy. Bacteria and toxins can act from outside cells, whilst viruses must enter cells to multiply and damage them from within.
Students often think all pathogens must enter cells to cause disease. This is incorrect—bacteria can damage cells from outside by releasing toxins, whilst viruses must enter cells to reproduce.
Think of bacterial toxins like arrows fired at a castle (the body)—they damage it from a distance. Viruses are like invaders that must get inside the castle to take over and cause damage from within.
Section 3
How are communicable diseases spread?
Communicable diseases are spread from one organism to another through four main transmission routes:
1. Direct contact
- Disease spread through touching infected person's skin or body fluids
- Examples: Gonorrhoea (sexually transmitted), skin infections (fungi)
- Prevention: Avoid touching infected individuals; use protective barriers
2. Water
- Pathogens transmitted via contaminated drinking water or food
- Example: Salmonella (spread through contaminated food or water)
- Prevention: Clean water supply; proper food hygiene; sewage treatment
3. Air
- Pathogens spread as droplets when infected person coughs or sneezes
- Example: Measles, tuberculosis (spread via respiratory droplets)
- Prevention: Face masks; isolation of infected people; ventilation
4. Vectors
- Animals that carry and transmit pathogens without being harmed
- Example: Mosquitoes transmit malaria parasites by biting infected and uninfected humans
- Prevention: Insecticides; bed nets; avoiding mosquito bites in endemic areas
Understanding transmission routes is essential for controlling disease spread in populations.
Examiners test your ability to match diseases to their transmission route. Know that measles spreads by air, malaria by vectors, Salmonella by water, and gonorrhoea by direct contact.
If asked how to prevent malaria spread in a tropical region: identify mosquitoes as vectors, then suggest controls like insecticide spraying, bed nets, and removing standing water where mosquitoes breed.
Section 4
What are the specific diseases caused by different pathogens?
Bacterial diseases:
| Disease | Cause | Transmission | Symptoms |
|---|---|---|---|
| Tuberculosis (TB) | Mycobacterium tuberculosis bacterium | Air (droplets from coughing) | Persistent cough, chest pain, fever, weight loss |
| Salmonella | Salmonella bacterium | Water/contaminated food | Diarrhoea, stomach cramps, vomiting |
| Gonorrhoea | Neisseria gonorrhoeae bacterium | Direct contact (sexual) | Genital discharge, pain during urination; often asymptomatic |
Viral diseases:
| Disease | Cause | Transmission | Symptoms |
|---|---|---|---|
| Measles | Measles virus | Air (respiratory droplets) | Rash, fever, cough, runny nose |
| HIV | Human Immunodeficiency Virus | Blood/body fluids (sexual contact, blood transfusion) | Initially flu-like; progresses to AIDS if untreated |
| Tobacco Mosaic Virus (TMV) | TMV particle | Direct contact (handling plants); sap | Mottled/mosaic pattern on leaves; reduced growth |
Fungal diseases:
| Disease | Cause | Transmission | Symptoms |
|---|---|---|---|
| Rose black spot | Diplocarpon rosae fungus | Water/air (spores) | Black or brown spots on leaves; leaf drop |
Protist diseases:
| Disease | Cause | Transmission | Symptoms |
|---|---|---|---|
| Malaria | Plasmodium (protist parasite) | Vector (mosquito bites) | Fever, chills, headache, muscle pain; recurring cycles |
Each pathogen type causes characteristic symptoms depending on the organs/tissues it affects.
Learn the specific pathogen, transmission route, and one or two key symptoms for each disease. Examiners ask 'Name the pathogen that causes...' so you must use correct scientific names (e.g., Mycobacterium tuberculosis, not just 'TB bacterium').
Section 5
How can the spread of communicable diseases be prevented or reduced?
Disease prevention strategies target transmission routes and depend on the disease. Key prevention methods include:
1. Breaking the transmission chain
- Isolate infected individuals to prevent person-to-person spread
- Example: Quarantine for measles or tuberculosis patients
2. Improving hygiene and sanitation
- Clean water supply and sewage treatment (prevents water-borne diseases like Salmonella)
- Hand washing and food preparation hygiene
- Personal protective equipment (barriers) for sexually transmitted infections
3. Vaccination
- Immunise populations against viral diseases (measles, HIV vaccine development)
- Builds herd immunity, reducing disease spread
4. Vector control
- Insecticides to kill mosquitoes (malaria prevention)
- Bed nets impregnated with insecticide
- Drain standing water to reduce mosquito breeding sites
- Use of repellents
5. Antibiotic treatment
- Treat bacterial infections early (TB, Salmonella, gonorrhoea)
- Reduces duration of infection and spread to others
6. Education and public health
- Awareness campaigns about transmission routes
- Safe sexual practices to reduce STI spread
- Respiratory hygiene (covering coughs/sneezes)
For plant diseases:
- Remove infected plants (rose black spot)
- Use fungicides on plants
- Improve air circulation and reduce humidity
Effective disease control requires a combination of these strategies tailored to the specific pathogen and transmission route.
Examiners ask how to prevent or reduce spread of specific diseases. Match prevention methods to transmission routes: e.g., for TB (air spread) suggest isolation; for malaria (vector) suggest insecticides and bed nets; for Salmonella (water) suggest clean water supply.
To prevent measles: vaccinate children (builds herd immunity and blocks air transmission). To prevent malaria in endemic regions: distribute insecticide-treated bed nets and spray swamps (targets the mosquito vector). To prevent gonorrhoea: use condoms and practice safe sex (breaks direct contact transmission).
Must Know
- Pathogens are microorganisms causing disease; four types are bacteria (prokaryotic, binary fission), viruses (non-living, require host cells), fungi (eukaryotic, form spores), and protists (eukaryotic, single-celled)
- Pathogens cause disease by producing toxins (damage from outside cells) or directly damaging cells (especially viruses entering and destroying cells)
- Four transmission routes: direct contact (touching), water (contaminated food/drink), air (respiratory droplets), vectors (insect bites—malaria mosquitoes)
- Key diseases: TB and measles spread by air; Salmonella by water; gonorrhoea by direct contact; malaria by mosquito vectors; rose black spot by fungal spores; HIV by blood/body fluids
- Prevention strategies include isolation, vaccination, hygiene/sanitation, vector control (insecticides, bed nets), antibiotics for bacterial infections, and education about safe practices
- Match specific prevention methods to transmission routes: isolation for air-spread diseases, clean water for water-borne diseases, contraception for STIs, insecticides for vector-borne diseases
That's the notes covered.
Carry on to the next subtopic.