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1. Communicable Diseases

Specification Point

Know what communicable diseases are, how they are transmitted, and how different pathogens cause disease.

Definition

A communicable disease is:

They are caused by pathogens.

Examples include:

• Influenza

• Tuberculosis (TB)

• HIV/AIDS

• Malaria

• Athlete's foot

• Tobacco Mosaic Virus (plants)

Pathogen

A pathogen is:

OCR requires you to know four groups:

Not all microorganisms are pathogens

Many microorganisms are useful.

Examples:

• Gut bacteria help digestion.

• Yeast is used in bread making.

• Bacteria recycle nutrients in ecosystems.

Only some microorganisms are pathogenic.

How Pathogens Cause Disease

Different pathogens damage the body in different ways.

Viruses:

• Enter host cells.

• Replicate inside them.

• Burst or damage cells.

Bacteria:

• Produce toxins.

• Damage tissues.

• Multiply rapidly.

Fungi:

• Digest living tissue.

• Damage cells with enzymes.

Protoctists:

• Invade tissues.

• Feed on host cells.

• Destroy red blood cells (malaria).

Transmission of Disease

Transmission = movement of pathogens between hosts.

OCR divides transmission into:

Direct Transmission

Spread by physical contact.

Examples:

• Touching

• Kissing

• Sexual intercourse

• Blood transfusions

• Needle sharing

Diseases:

• HIV

• Gonorrhoea

• Athlete's foot (skin contact)

Indirect Transmission

Spread without direct contact.

Examples:

• Air droplets

• Contaminated food

• Contaminated water

• Surfaces (fomites)

Diseases:

• Influenza

• Tuberculosis

• Food poisoning

Vector Transmission

A vector carries pathogens between hosts.

The vector is usually unaffected.

Example:

Female Anopheles mosquito
⬇
Carries Plasmodium
⬇
Causes malaria

Other vectors include:

• Ticks

• Fleas

• Aphids (plants)

Factors Affecting Disease Spread

Factors include:

Population density

• More people = more contact.

Poor sanitation

• Contaminated water spreads pathogens.

Travel

• Diseases move between countries quickly.

Climate

• Warm climates favour mosquito breeding.

Vaccination rates

• Low vaccination increases spread.

Poor nutrition

• Weakens immune system.

Age

• Babies and elderly often have weaker immunity.

Epidemic vs Pandemic

Epidemic

• Rapid spread within one country or area.

Pandemic

• Worldwide epidemic.

Example:
COVID-19 pandemic.

Reducing Disease Transmission

Methods include:

• Vaccination

• Hand washing

• Safe sex

• Isolation of infected individuals

• Mosquito control

• Clean drinking water

• Food hygiene

• Sterilising equipment

2. Types of Pathogen

Viruses

Definition

Viruses are acellular, non-living infectious particles.

They can only reproduce inside living cells.

They are called obligate intracellular parasites.

Structure

Viruses consist of:

• DNA or RNA

• Protein coat (capsid)

• Sometimes a lipid envelope

• Attachment proteins

They have:

❌ No cytoplasm

❌ No ribosomes

❌ No nucleus

❌ No mitochondria

Why Viruses Are Different

Viruses cannot:

• Respire

• Grow

• Divide

• Carry out metabolism

Outside cells they are inactive.

Bacteria

Definition:

Single-celled prokaryotes.

Most are harmless.

Some are pathogenic.

Structure

Contain:

• Cell wall (peptidoglycan)

• Cell membrane

• Cytoplasm

• Ribosomes (70S)

• Circular DNA

• Plasmids

May have:

• Capsule

• Flagellum

• Pili

Reproduction

Binary fission.

One bacterium
↓

DNA replicates
↓

Cell elongates
↓

Cell divides
↓

Two identical bacteria

Can reproduce every 20 minutes in ideal conditions.

Fungi

Fungi are eukaryotic organisms.

Examples:

• Athlete's foot

• Ringworm

Structure

Made of:

Hyphae

↓

Network called mycelium

Hyphae release enzymes that digest tissue externally.

Nutrients are absorbed afterwards.

Reproduction

Usually produce spores.

Spores spread by:

• Wind

• Water

• Animals

Protoctists

Mostly single-celled eukaryotes.

Some photosynthesise.

Some are parasites.

OCR example:

Plasmodium

Structure

Contain:

✔ Nucleus

✔ Cytoplasm

✔ Cell membrane

✔ Mitochondria

Some possess chloroplasts.

Comparison Table

Memory Trick

Virus = Invades.

Bacteria = Binary fission.

Fungi = Hyphae.

Protoctists = Parasites.

Summary

Viruses are not cells.

Bacteria are prokaryotes.

Fungi digest externally.

Protoctists are eukaryotic parasites.

3. Viral Diseases

General Characteristics

Viruses infect every type of organism.

They infect:

• Animals

• Plants

• Bacteria

Viral Replication

Diagram:

DNA/RNA in virus

↓

Attachment to host cell

↓

Virus enters cell

↓

Genetic material released

↓

Host cell makes viral proteins & nucleic acid

↓

New viruses assembled

↓

Cell bursts (lysis) or viruses leave

↓

More cells infected

Key point:
Viruses use the host cell's ribosomes and enzymes because they do not have their own.

OCR Example 1 – HIV

(Human Immunodeficiency Virus)

Transmission

• Unprotected sex

• Blood transfusions

• Shared needles

• Mother to baby (pregnancy, birth or breastfeeding)

Not spread by:

• Hugging

• Sharing food

• Coughing

• Casual contact

Target Cells

HIV infects T helper (CD4) cells, which coordinate the immune response.

Life Cycle (Simplified)

1. Virus attaches to T helper cell.

2. Viral RNA enters the cell.

3. Reverse transcriptase makes DNA from viral RNA.

4. Viral DNA is inserted into the host DNA.

5. Host cell makes new viruses.

6. Viruses leave to infect more T helper cells.

Loss of T helper cells weakens the immune system over time.

HIV vs AIDS

HIV = the virus.

AIDS (Acquired Immune Deficiency Syndrome) = the late stage of infection, where the immune system is severely weakened and opportunistic infections become common.

Treatment

There is currently no cure, but antiretroviral therapy (ART) can reduce viral replication, lower the viral load and allow people with HIV to live long, healthy lives.

OCR Example 2 – Influenza

Cause:
Influenza virus.

Transmission

• Droplets from coughing or sneezing

• Contaminated surfaces

Symptoms

• Fever

• Muscle aches

• Fatigue

• Headache

• Cough

• Sore throat

Why Flu Vaccines Change Every Year

The influenza virus mutates frequently.

Changes to its surface antigens mean previous antibodies may not recognise new strains effectively, so vaccines are updated regularly.

OCR Example 3 – Tobacco Mosaic Virus (TMV)

TMV infects plants.

Common hosts include:

• Tobacco

• Tomatoes

• Peppers

Symptoms

• Mosaic (patchy) leaf pattern

• Yellowing

• Reduced chlorophyll

• Reduced photosynthesis

• Stunted growth

• Lower crop yield

Transmission

• Contaminated tools

• Infected plant material

• Handling infected plants

Economic Importance

TMV reduces crop yield and quality, causing financial losses for farmers.

Why Viral Diseases Are Difficult to Treat

Viruses live inside host cells.

Many antiviral drugs can also damage host cells because viral replication uses the host's machinery.

Vaccination is therefore an important method of prevention.

Summary

HIV

• Targets T helper cells.

• Weakens the immune system.

• Spread by body fluids.

Influenza

• Respiratory virus.

• Mutates rapidly.

• Annual vaccines.

TMV

• Plant virus.

• Reduces photosynthesis.

• Lowers crop yield.

4. Bacterial Diseases

Specification Point

Know the structure of bacteria, how they reproduce, how they cause disease, and examples of bacterial diseases.

What are Bacteria?

Bacteria are single-celled prokaryotic organisms.

Most bacteria are harmless or beneficial, but some are pathogenic.

Examples of beneficial bacteria:

• Lactobacillus in yoghurt production

• Gut bacteria that produce vitamins and aid digestion

• Decomposer bacteria that recycle nutrients

Structure of a Bacterial Cell

A typical bacterial cell contains:

• Cell wall made of peptidoglycan (murein)

• Plasma membrane

• Cytoplasm

• 70S ribosomes

• Circular DNA (naked DNA)

• Plasmids (small loops of DNA)

Some bacteria also have:

• Capsule (protection from the immune system)

• Flagellum (movement)

• Pili (attachment and DNA transfer)

Exam Tip

Never say bacteria have a nucleus or membrane-bound organelles—they do not.

Binary Fission

Bacteria reproduce by binary fission.

Steps:

1. Circular DNA replicates.

2. Cell grows larger.

3. DNA copies move to opposite ends.

4. Cytoplasm divides.

5. Cell wall forms.

6. Two genetically identical daughter cells are produced.

Under ideal conditions, some bacteria divide every 20 minutes, causing rapid population growth.

Growth Curve (Useful Knowledge)

When bacteria are grown in culture, they show four phases:

1. Lag phase – Cells adapt to new conditions.

2. Log (Exponential) phase – Rapid cell division.

3. Stationary phase – Nutrients become limited; birth rate = death rate.

4. Death phase – More bacteria die than reproduce.

OCR may link this to required practicals on culturing microorganisms.

How Bacteria Cause Disease

Unlike viruses, bacteria usually do not need to enter cells.

They cause disease by:

1. Producing Toxins

Toxins are poisonous chemicals that:

• Damage cells

• Disrupt enzyme activity

• Destroy tissues

• Cause inflammation

2. Damaging Body Tissues

Some bacteria produce enzymes that digest host tissues, allowing them to spread.

3. Rapid Multiplication

Large bacterial populations produce increasing amounts of toxins and overwhelm the body's defences.

OCR Example – Tuberculosis (TB)

Cause:
Mycobacterium tuberculosis

Transmission

Spread by:

• Airborne droplets from coughing or sneezing

Risk increases with:

• Overcrowding

• Poor ventilation

• Weakened immune systems

Symptoms

• Persistent cough (lasting several weeks)

• Chest pain

• Fever

• Night sweats

• Weight loss

• Fatigue

• Coughing up blood (in severe cases)

Effects on the Body

TB mainly infects the lungs.

The immune system forms tubercles (small nodules) around infected cells.

These can reduce lung function by damaging lung tissue.

Prevention

• Early diagnosis

• Antibiotic treatment

• Isolation of infectious patients

• Improved living conditions

• BCG vaccination (used in some countries)

OCR Example – Gonorrhoea

Comparison of TB and Gonorrhoea

5. Fungal Diseases

Specification Point

Know how fungi cause disease and examples of fungal infections.

What are Fungi?

Fungi are eukaryotic organisms.

They may be:

• Unicellular (e.g. yeast)

• Multicellular (e.g. moulds)

Pathogenic fungi invade tissues and absorb nutrients from living organisms.

Structure of Fungi

Multicellular fungi consist of:

Hyphae → long thread-like filaments.

Many hyphae together form a mycelium.

The mycelium has a large surface area, increasing nutrient absorption.

How Fungi Feed

Fungi use extracellular digestion.

Steps:

1. Hyphae release digestive enzymes.

2. Enzymes break down surrounding tissue.

3. Small molecules are absorbed into the fungus.

This damages host cells and causes disease.

Reproduction

Many fungi reproduce using spores.

Spores are:

• Tiny

• Resistant

• Easily dispersed

They spread by:

• Wind

• Water

• Animals

• Human contact

OCR Example – Athlete's Foot

Cause:
Dermatophyte fungus.

Transmission

• Walking barefoot on contaminated floors

• Sharing towels

• Sharing shoes

• Warm, damp environments

Symptoms

• Itchy skin

• Cracked skin

• Redness

• Peeling skin

• Blisters

Prevention

• Keep feet dry

• Wear flip-flops in communal showers

• Do not share towels

• Wear clean socks

Treatment

Antifungal creams or sprays.

OCR Example – Ringworm

Despite its name, ringworm is not caused by a worm.

It is a fungal infection.

Symptoms

• Circular red rash

• Itching

• Flaking skin

• Hair loss if scalp is infected

Transmission

• Direct contact

• Pets

• Shared clothing

• Shared brushes

6. Protoctist Diseases

Specification Point

Know that some protoctists are parasites and understand malaria.

What are Protoctists?

Protoctists are eukaryotic organisms.

Some:

• Photosynthesise

• Feed on other organisms

• Live as parasites

OCR focuses on Plasmodium, which causes malaria.

OCR Example – Malaria

Cause:
Plasmodium protoctist.

Vector:
Female Anopheles mosquito.

Transmission

1. Mosquito bites infected person.

2. Mosquito takes up Plasmodium.

3. Parasite develops inside mosquito.

4. Mosquito bites another person.

5. Parasite enters bloodstream.

Simplified Life Cycle

Mosquito bite

↓

Parasites enter blood

↓

Travel to liver

↓

Multiply

↓

Enter red blood cells

↓

Multiply again

↓

Red blood cells burst

↓

More parasites released

↓

Another mosquito bites infected person

↓

Cycle repeats

Symptoms

• High fever

• Chills

• Sweating

• Fatigue

• Headaches

• Anaemia (due to destruction of red blood cells)

Severe malaria can damage organs and become life-threatening.

Prevention

• Insecticide-treated mosquito nets

• Insect repellents

• Removing standing water

• Indoor insecticide spraying

• Antimalarial medication

• Rapid diagnosis and treatment

Why Malaria is Difficult to Control

• Mosquitoes breed quickly.

• Plasmodium has a complex life cycle.

• Some mosquitoes are resistant to insecticides.

• Some Plasmodium strains are resistant to antimalarial drugs.

• No single control method is completely effective.

7. Comparison of Major Human Pathogens

8. Plant Diseases

Specification Point

Know examples of plant diseases and their effects on crops.

Plants can be infected by:

• Viruses

• Bacteria

• Fungi

Plant diseases reduce:

• Growth

• Photosynthesis

• Crop yield

• Food production

• Farmer income

OCR Example – Tobacco Mosaic Virus (Revision)

Symptoms:

• Mosaic leaf pattern

• Yellow leaves

• Reduced chlorophyll

• Less photosynthesis

• Reduced crop yield

Transmission:

• Contaminated tools

• Infected plants

• Human handling

OCR Example – Black Sigatoka

Cause:
Fungus.

Affects:
Banana plants.

Symptoms

• Dark streaks on leaves

• Yellowing

• Premature leaf death

• Reduced photosynthesis

• Smaller bananas

• Lower yields

Economic Importance

Large financial losses for banana producers.

Can increase food prices.

Control

• Fungicides

• Resistant banana varieties

• Removing infected leaves

• Good plantation management

OCR Example – Ring Rot

Cause:
Bacterium.

Affects:
Potatoes.

Symptoms

• Rotting vascular tissue

• Yellow leaves

• Wilting

• Reduced growth

• Potato tubers rot internally

Transmission

• Contaminated equipment

• Infected seed potatoes

• Soil

Prevention

• Destroy infected crops

• Sterilise farming equipment

• Use certified disease-free seed potatoes

• Crop monitoring

Comparing Plant Diseases

Economic Effects of Plant Diseases

Plant diseases can:

• Reduce crop yield.

• Reduce food quality.

• Increase food prices.

• Reduce farmers' profits.

• Increase pesticide use.

• Threaten food security.

9. The Immune System

Specification Overview

You must understand:

• The body’s defence systems

• Non-specific and specific immunity

• How white blood cells respond to pathogens

A) Types of Immunity

Non-specific immunity (innate)

Acts the same way against all pathogens.

Includes:

• Physical barriers

• Chemical barriers

• Phagocytosis

• Inflammation response

Specific immunity (adaptive)

Targets specific antigens on pathogens.

Involves:

• T lymphocytes

• B lymphocytes

• Antibodies

• Memory cells

B) Physical Barriers

These prevent pathogen entry:

• Skin (physical barrier)

• Mucus (traps pathogens)

• Cilia in airways (move mucus out)

• Stomach acid (kills pathogens)

• Tears (contain enzymes)

C) Phagocytosis

Phagocytosis is the process by which white blood cells engulf pathogens.

Steps of Phagocytosis:

1. Pathogen is detected by phagocyte

2. Phagocyte moves towards pathogen (chemotaxis)

3. Phagocyte surrounds pathogen

4. Pathogen is engulfed into a vesicle (phagosome)

5. Lysosomes fuse with phagosome

6. Enzymes break down pathogen

7. Antigens from pathogen are displayed on cell surface

Key term:

Antigen = a molecule on a pathogen that triggers an immune response

D) Antigen Presentation

After digestion, phagocytes:

• Display antigens on their surface

• Act as antigen-presenting cells (APCs)

This activates T lymphocytes.

E) Inflammation Response

When tissue is damaged:

• Histamine is released

• Blood vessels dilate

• Capillaries become more permeable

• White blood cells move into tissue

Results:

• Redness

• Swelling

• Heat

• Pain

10. T Lymphocytes

T cells are involved in cell-mediated immunity.

They mature in the thymus gland.

Types of T Cells

1. T Helper Cells

Function:

• Activate B cells

• Activate T killer cells

• Release cytokines

They coordinate the immune response.

2. T Killer Cells (Cytotoxic T cells)

Function:

• Destroy infected body cells

• Release toxins that cause cell death

Important for:

• Virus-infected cells

• Cancer cells

3. T Memory Cells

Function:

• Remain in bloodstream long-term

• Provide rapid response on reinfection

Responsible for secondary immune response.

4. T Regulatory Cells

Function:

• Suppress immune response

• Prevent immune system attacking self cells

• Maintain immune balance

Cell-Mediated Immunity

Steps:

1. Pathogen infects body cell

2. Antigens displayed on infected cell

3. T helper cells detect antigen

4. T helper cells activate T killer cells

5. T killer cells clone themselves

6. Killer cells destroy infected cells

7. Memory T cells remain

Key idea:

This response targets infected body cells, not free pathogens.

11. B Lymphocytes

B cells are involved in humoral immunity (antibody-mediated immunity).

They mature in the bone marrow.

Clonal Selection

Steps:

1. Specific antigen binds to a B cell receptor

2. Only matching B cell is activated

3. B cell divides by mitosis (clonal expansion)

4. Forms:

   • Plasma cells

   • Memory cells

Plasma Cells

Function:

• Produce large quantities of antibodies

• Short-lived cells

• Act immediately during infection

Memory B Cells

Function:

• Remain in body long-term

• Produce faster response upon reinfection

• Basis of vaccination

Humoral Immunity Summary

Steps:

1. Antigen enters body

2. B cell with complementary receptor binds antigen

3. T helper cell activates B cell

4. B cell clones itself

5. Plasma cells produce antibodies

6. Memory cells remain

Primary vs Secondary Response

Primary Response:

• First exposure

• Slow

• Low antibody production

Secondary Response:

• Second exposure

• Rapid

• Much higher antibody production

• Stronger immunity

Comparison: T Cells vs B Cells

12.