Communicable (infectious) diseases & Viral, Bacterial, Fungal, Protist diseases

SPEC 8461 • MODULE 3 • LESSON 3.01

Communicable Diseases & How Pathogens Spread

The four pathogen types, the four spread routes, and the mark-scheme verbs that win the marks examiners reward.

The Mark Scheme That Ignores "Kissing"

HIV transmission questions appear repeatedly across AQA Paper 1 — and the mark scheme is consistent every time it appears.

Examiners accept "sexual contact / intercourse" or "exchange of body fluids" (with named examples like sharing needles). They explicitly ignore "kissing" as a transmission route. — Recurring AQA Mark-Scheme Pattern

Why This Lesson

"Kissing" is a common student answer — and it scores zero. In the next 10 minutes, you'll know the four pathogen types, how each spreads, and the credit-worthy named routes that win marks every time.

Learning Objectives

By the end of this lesson, you'll be able to:

Objective 1

Identify the four types of pathogen — bacteria, viruses, fungi, protists — and state a spec-named example for each.

Objective 2

Describe how each pathogen can spread using the four named transmission routes — direct contact, water, air (droplets), and vectors.

Objective 3

Explain how to reduce or prevent the spread of disease using hygiene, isolation, vector destruction, and vaccination.

Spec ref: 4.3.1.1 · Paper: 1 · Tier: Higher

What Is a Pathogen?

Pathogen

A microorganism (bacterium, virus, fungus, or protist) that causes infectious disease.

Communicable (Infectious) Disease

A disease caused by a pathogen that can spread from one organism to another. The spread is what makes it communicable.

Spec-Critical Detail

The spec lists exactly four types of pathogen. Pathogens may infect plants or animals and can be spread by direct contact, by water, or by air. Both bacteria and viruses may reproduce rapidly inside the body.

The Four Pathogen Types

Memorise all four with a spec-named example for each. Pathogens infect both plants and animals — and the mark-scheme verbs differ for each type.

Bacteria

Single-celled prokaryotes.

Damage: release / produce poisons (toxins) that damage tissues.

Examples: Salmonella · gonorrhoea

Viruses

Not cellular; reproduce inside host cells.

Damage: enter cells, reproduce, then burst the cell.

Examples: measles · HIV · TMV (plant)

Fungi

Some are filament-like (hyphae); some are unicellular (yeasts).

Damage: damage tissues; spores spread by air or water.

Example: rose black spot (plant)

Protists

Single-celled eukaryotes.

Damage: often transferred to host by a vector.

Example: malarial protist (Plasmodium) — mosquito vector

Memory Hook — BVFP

Bacteria release toxins · Viruses burst cells · Fungi spread spores · Protists need a vector. Four types, four mechanisms — one mark-scheme verb each.

How Bacteria Cause Disease

Bacteria reproduce rapidly by binary fission. Once inside the body, they cause symptoms by a specific mechanism — and the mark scheme is strict about the verb.

Bacteria release / produce toxins → toxins damage cells and tissues

1

Bacteria enter the body and reproduce rapidly inside it.

2

Bacteria release / produce toxins (poisons) into the body.

3

Toxins damage cells and tissues.

4

This causes the symptoms of the disease (fever, vomiting, diarrhoea).

Mark-Scheme Verb

AQA mark schemes credit "(bacteria) release / produce poisons (toxins)" — the action verb is the mark. They ignore "toxins unqualified": "they have toxins" or "make you ill" both score zero.

How Viruses Cause Disease

Viruses can't reproduce on their own. They need a host cell — and they destroy it on the way out.

The viral lysis cycle — each new virus infects more cells

1

Virus enters a host cell.

2

Virus uses the cell's machinery to make many copies of itself.

3

The cell bursts, releasing the new viruses.

4

Each new virus infects more cells — the disease spreads through the body and causes symptoms.

Exam Tip

The mark-scheme phrasing: viruses "live and reproduce inside cells, causing cell damage." Don't write "viruses are alive" or "viruses kill cells" alone — name the mechanism: reproduce inside, then burst the cell.

Fungi & Protists — The Two Often-Forgotten Types

Most students remember bacteria and viruses. Fungi and protists are where the easier marks are lost.

Fungi — Rose Black Spot

A fungal plant disease. The spec's named example.

→ Causes purple or black spots on rose leaves

→ Leaves can turn yellow and drop early

→ Reduces photosynthesis, weakening plant growth

→ Spread by air and water (spores)

→ Treated with fungicides; remove infected leaves

Protists — Malaria

A protist disease spread by a vector.

→ Caused by the malarial protist (Plasmodium)

→ Spread by the mosquito vector

→ Causes recurrent episodes of fever; can be fatal

→ Prevention: destroy mosquito breeding sites; use mosquito nets to prevent bites

Don't Confuse

A vector is an organism that carries the pathogen — the mosquito carries the protist; the mosquito is not itself the cause of malaria. Mark schemes credit "vector" or "mosquito" — not "insect".

Vector transmission — mosquito carries the malarial protist between hosts

How Pathogens Spread

The spec names three main routes: direct contact, water, and air. Vectors are introduced separately as the spread route for malaria. Know all four.

Direct Contact

Skin-to-skin contact, sexual contact, or pathogen entering through cuts.

Examples: gonorrhoea (sexual contact); plant-to-plant transfer of TMV (via contaminated tools)

Water

Drinking water contaminated with pathogens (especially bacteria).

Spec note: water is named as a route, but no water-only disease is named at 4.3.1.1

Air (Droplets)

Inhaling droplets from coughing and sneezing.

Example: measles (viral); also fungal spores in air

Vectors (Special Case)

An organism carries the pathogen to a host.

Example: malaria — mosquito carries the protist

Exam Tip

Some pathogens spread by multiple routes. HIV spreads only through exchange of body fluids — covered separately on the next slide.

HIV — The Mark-Scheme Trap

HIV spreads through exchange of body fluids. The mark scheme is specific about what counts and what doesn't.

✓ ACCEPTED: sexual contact / intercourse

✓ ACCEPTED: exchange of body fluids (with named examples)

✓ ACCEPTED: sharing needles · mother to baby (pregnancy / birth / breastfeeding) · blood transfusion

✗ IGNORED: kissing

✗ IGNORED: touching (too vague)

Common Zero-Mark Answer

"Kissing" is a common student answer that scores zero. When asked for one way HIV spreads, pick one specific named route — don't list multiple, and don't write "kissing".

How to Reduce or Prevent Spread

The spec asks students to explain how spread can be reduced — not just name measures. Pair each measure with why it works.

MeasureWhy It Works (mark-scheme reasoning)

HygieneHandwashing, food preparation, personal cleanliness — removes pathogens before they enter the body or contaminate food.

Isolating infected individualsPrevents direct contact and droplet spread to healthy people.

Destroying vectorse.g. draining stagnant water kills mosquito larvae ; insecticides kill adult mosquitoes — breaks the transmission chain for malaria.

VaccinationImmunises the population so pathogens have fewer hosts to infect — covered in Lesson 3.05.

Exam Tip

Generic answers like "wash your hands" can score — but specifying why ("to remove bacteria before eating") earns the extra explain mark. Match length to mark allocation.

Worked Example — Step 1 & 2

"Give one method by which HIV can be transmitted between people." — Exam-style question · [1 mark]

Step 1 — Read the question

Command word: "Give" — recall, no reasoning needed. Mark allocation: 1 mark. Quantity: one way.

Step 2 — Identify the pattern

This is a 1-mark recall question. Single creditable answer. Multiple variants are acceptable, so you only need one specific named route — picking more is a waste of writing time and risks listing a wrong one.

Pattern 1 Rule

When multiple answers are acceptable, pick one specific named route. Don't list multiple — examiners only mark the first credit-worthy answer, but listing a wrong one alongside can cost the mark.

Worked Example — Steps 3, 4 & 5

Step 3 — Plan the answer

One creditable route. Mark scheme accepts: "sexual contact / intercourse" OR "exchange of body fluids" (with examples like sharing needles, blood transfusion, mother to foetus).

✗ 0 marks — "kissing"

"HIV spreads by kissing."

Explicitly ignored by the mark scheme. Common answer, common zero.

✓ 1 mark

"HIV can be transmitted through sexual contact."

OR: "Through exchange of body fluids, e.g. sharing needles."

Step 5 — Match to mark scheme

Mark scheme: "sexual contact / intercourse" OR "exchange of body fluids" (allow named examples). ✓ Mark awarded.

Three Examiner Alerts

Alert 1 — HIV transmission

For HIV, write "sexual contact" or "exchange of body fluids" (with named examples). AQA mark schemes consistently ignore "kissing" — it's not a credit-worthy route.

Alert 2 — Bacterial mechanism

When describing how bacteria cause disease, write "release / produce toxins" — the action verb matters. AQA mark schemes ignore "toxins unqualified" (e.g. "bacteria have toxins" scores zero; "bacteria release toxins" wins the mark).

Alert 3 — Name the specific pathogen

Use the spec-named examples: Bacteria → Salmonella, gonorrhoea. Viruses → measles, HIV, TMV. Fungi → rose black spot. Protists → malarial protist. Don't generalise — name them.

Common Errors — Wrong vs Right

Four high-frequency errors from past papers. Each one swaps a vague phrase for the mark-scheme version.

✗ Wrong (loses marks)✓ Right (mark-scheme version)

"HIV spreads by kissing""HIV spreads by sexual contact or exchange of body fluids "

"bacteria make you ill""bacteria release / produce toxins which damage cells"

"viruses are alive cells""viruses are not cellular ; they reproduce inside living cells and burst them"

"all pathogens are bacteria""four types — bacteria, viruses, fungi, protists "

Recap — Three Takeaways

1

Four pathogen types. Bacteria (release toxins) · viruses (reproduce in cells, burst them) · fungi (rose black spot) · protists (malarial protist via mosquito vector).

2

Four routes of spread. Direct contact · water · air (droplets) · vectors. HIV is "exchange of body fluids" — mark scheme ignores "kissing".

3

Prevention measures. Hygiene · isolating infected individuals · destroying vectors · vaccination. Specifying why each works earns more marks.

Exam Tip

Three things. You're set for any pathogen-spread question on Paper 1. Pair every named pathogen with its mechanism verb, and every prevention measure with a "so that…" clause.

Summary

Key Points

A pathogen is a microorganism causing infectious disease — four types: bacteria, viruses, fungi, protists

Bacteria release / produce toxins · viruses reproduce inside cells and burst them

Four routes of spread: direct contact, water, air (droplets), vectors

HIV: exchange of body fluids (mark scheme ignores "kissing" )

Prevention: hygiene · isolation · destroy vectors · vaccination

Named Examples

Bacteria: Salmonella, gonorrhoea

Viruses: measles, HIV, TMV

Fungi: rose black spot

Protists: malarial protist

Mark-Scheme Verbs

Bacteria: release / produce toxins

Viruses: reproduce inside cells, burst the cell

HIV spread: exchange of body fluids

Malaria: spread by vector (mosquito)

Exam Tip

The single biggest mark loss on this topic is vague verbs. "Bacteria have toxins" loses the mark; "bacteria release toxins" wins it. The verb is the mark. Next: Lesson 3.02 — Bacterial and Viral Diseases (the named pathogens in detail).