Bacterial and Viral Disease — AQA GCSE Biology

SPEC 8461 • MODULE 3 • LESSON 3.02

Bacterial and Viral Diseases

Five named diseases — Salmonella, gonorrhoea, measles, HIV, TMV — and the mark-scheme verbs that separate full marks from zero.

The Verb That Wins the Mark

When AQA asks how Salmonella causes the symptoms of food poisoning, the mark scheme is razor-sharp on one detail.

The credit-worthy phrase is "(bacteria) release / produce toxins" — or the spec's own language, "the toxins they secrete". The mark scheme ignores "toxins unqualified": "bacteria have toxins" or "bacteria make you ill" scores zero. — Recurring AQA Mark-Scheme Pattern

Why This Lesson

This lesson covers the five spec-named diseases. For each, you need: symptoms · transmission · prevention — and the mark-scheme verb for the mechanism. Get the verb right; the mark follows.

Learning Objectives

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

Objective 1

Describe the two named bacterial diseases — Salmonella and gonorrhoea — including symptoms, spread, and prevention.

Objective 2

Describe the three named viral diseases — measles, HIV, and TMV — including symptoms, spread, and prevention.

Objective 3

Use mark-scheme language for how each pathogen causes symptoms — including the "release / produce toxins" verb for bacteria.

Spec ref: 4.3.1.2, 4.3.1.3 · Paper: 1 · Tier: Higher

Salmonella — Bacterial Food Poisoning

Spec: symptoms caused by both the bacteria AND the toxins they secrete

Spread & Prevention

Spread: bacteria ingested in food, or food prepared in unhygienic conditions.

Prevention (UK):poultry are vaccinated against Salmonella to control the spread; food hygiene reduces contamination.

What Causes the Symptoms

Spec is explicit: symptoms are caused by BOTH the bacteria AND the toxins they secrete. Use the spec verb "secrete" — or the mark-scheme alternatives "release / produce toxins".

Gonorrhoea — Bacterial STD

Gonorrhoea is a sexually transmitted disease (STD) caused by a bacterium. The spec uses it as the canonical example of antibiotic resistance.

FeatureSpec detail

SymptomsThick yellow or green discharge from the vagina or penis; pain on urinating .

SpreadBy sexual contact .

Original treatmentEasily treated with the antibiotic penicillin .

Problem nowMany strains are now resistant to penicillin — so other antibiotics are used.

PreventionTreatment with antibiotics; barrier methods of contraception (e.g. a condom) .

Exam Tip

Gonorrhoea is the spec's go-to example for explaining why antibiotic resistance matters. The full mechanism of resistance (natural selection) is covered in Lesson 3.05.

Measles — A Viral Disease

Measles is a viral disease. The spec emphasises that it can be serious or fatal — students must not write "measles is just a rash".

Symptoms

Fever + red skin rash.

Spec: measles is a serious illness that can be fatal if complications arise. Don't write "measles is just a rash".

Spread

By inhalation of droplets from sneezes and coughs.

Highly contagious — one of the reasons it's part of the routine childhood vaccination programme.

Prevention — Vaccination

Spec language: "most young children are vaccinated against measles". High vaccination rates protect the community by reducing pathogen circulation — covered in Lesson 3.03.

Don't Confuse

Measles is airborne (droplets), not foodborne. A common student error is writing that Salmonella spreads "by droplets" — that's how measles spreads, not Salmonella.

HIV — Viral Attack on the Immune System

HIV (human immunodeficiency virus) is a viral disease. Unless controlled, it can progress to AIDS — the late-stage condition.

Without antiretrovirals, immune cells decline → AIDS. With them, HIV is controlled.

Spread

Sexual contact, or exchange of body fluids such as blood — e.g. drug users sharing needles. Not spread by kissing.

Spec Mechanism Phrases

"HIV attacks the body's immune cells." Late stage = AIDS — immune system "so badly damaged it can no longer deal with other infections or cancers".

TMV — A Viral Plant Disease

Tobacco Mosaic Virus (TMV) is the spec's named example of a viral plant disease. The diagnostic feature is the leaf pattern.

TMV produces a distinctive mosaic pattern — patches reduce photosynthesis

Symptoms & Effect

Distinctive 'mosaic' pattern of discolouration on leaves. The patches reduce photosynthesis → affects plant growth.

Affected Plants

A widespread plant pathogen affecting many species — including tomatoes (named in spec).

All Five Diseases at a Glance

Bacterial

Salmonella

Spread: contaminated food

Symptoms: fever, vomiting, diarrhoea, cramps

Prevention: vaccinate poultry; cook food thoroughly

Bacterial

Gonorrhoea

Spread: sexual contact

Symptoms: yellow/green discharge, pain on urinating

Prevention: antibiotics (resistance growing); condoms

Viral

Measles

Spread: inhalation of droplets

Symptoms: fever + red skin rash

Prevention: vaccination (most young children)

Viral

HIV

Spread: sexual contact / body fluids (e.g. needles)

Symptoms: flu-like → attacks immune cells → AIDS (immune system can't deal with other infections or cancers)

Prevention/control: antiretroviral drugs

Viral (plant)

Tobacco Mosaic Virus (TMV)

Affects: many plant species including tomatoes

Symptoms: distinctive 'mosaic' pattern on leaves; reduces photosynthesis

Spec note: no specific prevention measures given in the spec for TMV

Mark-Scheme Verbs — Bacteria vs Viruses

The mechanism verb is the mark. Generic verbs ("make you ill") score zero; named mechanisms score full marks.

Bacteria

"Bacteria release / produce toxins"

OR spec language: "the toxins they secrete"

Toxins then damage cells and tissues, causing symptoms.

Viruses

"Viruses live and reproduce inside cells"

Then "burst the cell", causing cell damage.

For HIV: "attacks the body's immune cells".

✗ Generic (0 marks)✓ Mark-scheme verb

"bacteria have toxins""bacteria release / produce toxins"

"viruses kill cells""viruses reproduce inside cells and burst them "

"HIV makes you sick""HIV attacks the body's immune cells "

Worked Example — Steps 1 & 2

"Describe how Salmonella bacteria cause the symptoms of food poisoning in an infected person." — Exam-style question · [2 marks]

Step 1 — Read the question

Command word: "Describe" — sequential. 2 marks = 2 discrete points. Topic: Salmonella mechanism, not transmission.

Step 2 — Identify the pattern

2-mark mechanism question. Each mark = one named step in the cause-effect chain. Mark scheme likely accepts: (1) bacteria release / produce / secrete toxins, (2) toxins damage cells / cause vomiting + diarrhoea.

Mark Allocation Tip

Two marks ≠ two sentences of waffle. Two marks = two named mechanism steps. Pad with linker words ("which", "so that") to chain the steps, not to add filler.

Worked Example — Steps 3, 4 & 5

Step 3 — Plan

Two marks → two named steps. Step A: bacteria release toxins. Step B: toxins damage gut cells / cause symptoms.

✗ 0 marks

"Salmonella makes you ill because the bacteria are bad for your body."

No named mechanism. No action verb. No cause-effect chain.

✓ 2 marks

"Salmonella bacteria release toxins into the gut, which damage cells and cause the symptoms of vomiting and diarrhoea."

"release toxins" = 1 mark. "damage cells / cause symptoms" = 1 mark.

Step 5 — Match to mark scheme

Mark scheme accepts: "bacteria release / produce / secrete toxins" (1 mark) + "toxins damage cells / tissues" OR "cause vomiting / diarrhoea" (1 mark). ✓ Both marks awarded.

Three Examiner Alerts

Alert 1 — Salmonella verb

For Salmonella, write "release / produce / secrete toxins" — never "bacteria have toxins" or "make you sick". AQA mark schemes consistently ignore "toxins unqualified".

Alert 2 — Don't mix up spread routes

A recurring examiner note: students confuse Salmonella (foodborne) with measles (airborne droplets). Memorise the spread route by disease, not by pathogen type.

Alert 3 — HIV ≠ AIDS

HIV is the virus that infects the body. AIDS is the late-stage condition that develops if HIV is untreated. Writing "HIV is the same as AIDS" loses marks — both terms are spec-named separately.

Common Errors — Wrong vs Right

High-frequency errors on named-disease questions. Each one swaps a vague phrase for the mark-scheme version.

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

"Salmonella has toxins""Salmonella releases / produces / secretes toxins "

"measles spreads by touching""measles spreads by inhalation of droplets (sneezes/coughs)"

"HIV is the same as AIDS"" HIV is the virus; AIDS is the late-stage condition if HIV is untreated"

"TMV kills plants""TMV causes mosaic discolouration which reduces photosynthesis , affecting plant growth"

"gonorrhoea is cured by penicillin""gonorrhoea was treated with penicillin, but many strains are now resistant "

Recap — Three Takeaways

1

Bacterial diseases.Salmonella (food poisoning · releases toxins · vomiting + diarrhoea · poultry vaccinated). Gonorrhoea (STD · penicillin-resistant strains · condoms / antibiotics).

2

Viral diseases.Measles (fever + rash · droplets · MMR). HIV (body fluids · attacks immune cells · antiretrovirals → AIDS if untreated). TMV (plants · mosaic pattern · reduces photosynthesis).

3

Mark-scheme verbs. Bacteria → "release / produce / secrete toxins". Viruses → "reproduce inside cells and burst them". HIV → "attacks the body's immune cells". The action verb is the mark.

Exam Tip

Three things. You're set for any named-disease question on Paper 1. Pair every disease with its specific transmission route and its mark-scheme mechanism verb.

Summary

Key Points

2 bacterial diseases — Salmonella (food poisoning) · gonorrhoea (STD)

3 viral diseases — measles · HIV · TMV (plant)

Bacteria cause symptoms by secreting / releasing toxins that damage cells

Viruses cause symptoms by reproducing inside cells and bursting them

HIV specifically attacks immune cells ; late-stage condition is AIDS

TMV produces a mosaic discolouration on leaves that reduces photosynthesis

Spread Routes (Memorise)

Salmonella → food

Gonorrhoea → sexual contact

Measles → droplets (air)

HIV → body fluids (sex / needles)

TMV → plant-to-plant contact

Mark-Scheme Verbs

Bacteria: release / produce / secrete

Viruses: reproduce inside · burst

HIV: attacks immune cells

TMV: reduces photosynthesis

Exam Tip

The single biggest mark loss on these questions is using generic verbs. Every disease has its own mark-scheme mechanism — learn the specific verb for each. Next: Lesson 3.03 — Fungal and Protist Diseases (rose black spot, malarial protist).