Viral Diseases (HIV, Measles, TMV)

This lesson covers the three viral diseases named on the AQA GCSE Combined Science Trilogy specification (8464): measles, HIV and tobacco mosaic virus (TMV). You need to know the symptoms, how each is transmitted, and how it can be treated or prevented.

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How Viruses Cause Disease — Recap

Viruses are not true living cells. They consist of a strand of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. Because they have no cellular machinery, they must:

1. Attach to a host cell and inject their genetic material
2. Use the host cell's enzymes and ribosomes to replicate viral DNA/RNA and make new viral proteins
3. Assemble many new virus particles inside the host cell
4. The host cell bursts (lysis), releasing hundreds of new viruses

graph LR
    A[Virus attaches to host cell] --> B[Injects genetic material]
    B --> C[Host cell replicates viral DNA/RNA]
    C --> D[New virus particles assembled]
    D --> E[Host cell bursts — lysis]
    E --> F[New viruses infect more cells]
    F --> A

Exam Tip: The key word is lysis — the bursting of the host cell. This is what makes viral infections damaging.

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1. Measles

Feature	Detail
Pathogen	Measles virus (a paramyxovirus)
Symptoms	Fever, red skin rash, cough, runny nose
Transmission	Airborne — spread by droplet infection when an infected person coughs or sneezes
Complications	Can be fatal if complications develop, especially pneumonia or encephalitis (inflammation of the brain)
Prevention	MMR vaccine (measles, mumps, rubella) given to young children
Treatment	No specific treatment — the body's immune system must fight the virus; supportive care (rest, fluids)

Measles is highly contagious — one infected person can spread the virus to many others in enclosed spaces. This is why vaccination is so important: if enough people are vaccinated, herd immunity develops and the virus cannot spread easily.

Exam Tip: You should be able to explain why measles vaccination rates need to stay high. If vaccination rates fall, outbreaks can occur because the virus can spread through unvaccinated individuals.

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2. HIV (Human Immunodeficiency Virus)

Feature	Detail
Pathogen	Human Immunodeficiency Virus (HIV) — a retrovirus
Symptoms	Initial flu-like illness; then a long period with no symptoms while the virus attacks immune system cells (specifically T helper lymphocytes)
Transmission	Exchange of body fluids — sexual contact, sharing needles, or from mother to child during pregnancy/birth
Late-stage disease	If untreated, HIV destroys so many white blood cells that the immune system can no longer fight infections — this is called AIDS (Acquired Immune Deficiency Syndrome)
Prevention	Use of condoms, screening blood donations, not sharing needles, antiretroviral drugs during pregnancy
Treatment	Antiretroviral drugs — these do not cure HIV but can control the virus and prevent progression to AIDS

The Progression of HIV Infection

graph TD
    A[HIV enters the body] --> B[Initial flu-like illness]
    B --> C[Latent phase — no symptoms for years]
    C --> D[Virus slowly destroys T helper lymphocytes]
    D --> E[Immune system becomes severely weakened]
    E --> F[AIDS — body cannot fight other infections]
    F --> G[Opportunistic infections e.g. pneumonia, TB]

Exam Tip: Make sure you distinguish between HIV (the virus) and AIDS (the condition). A person can be HIV-positive for many years without having AIDS. AIDS only develops when the immune system is severely damaged.

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3. Tobacco Mosaic Virus (TMV)

Feature	Detail
Pathogen	Tobacco Mosaic Virus (TMV)
Host	Plants — especially tobacco plants, but also tomatoes, peppers, and other crops
Symptoms	Distinctive mosaic pattern of discolouration on leaves (patches of pale green, yellow, and dark green)
Effect on the plant	Discoloured patches mean less chlorophyll is present, which reduces the rate of photosynthesis and therefore reduces plant growth
Transmission	Direct contact between plants, contaminated hands or tools, insects
Prevention	Using resistant plant varieties, sterilising tools, good hygiene

Exam Tip: TMV is the only plant disease caused by a virus that you need to know for AQA Trilogy. Remember that it reduces photosynthesis because the mosaic pattern means less chlorophyll is available to absorb light.

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Comparing the Three Viral Diseases

Feature	Measles	HIV	TMV
Host	Humans	Humans	Plants
Main transmission	Airborne droplets	Body fluids	Direct contact / tools
Key symptom	Rash and fever	Weakened immune system	Mosaic leaf discolouration
Vaccine available?	Yes (MMR)	No	No (resistant varieties used)
Treatment	Supportive care only	Antiretroviral drugs	No cure
Can it be fatal?	Yes (complications)	Yes (AIDS)	No (reduces yield)

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Common Mistakes

Mistake	Correction
"HIV is the same as AIDS"	HIV is the virus; AIDS is the late-stage condition when the immune system fails
"Antibiotics can treat measles"	Measles is caused by a virus — antibiotics only work on bacteria
"TMV kills the plant immediately"	TMV reduces growth by reducing photosynthesis, but does not usually kill the plant outright
"HIV is spread by casual contact"	HIV requires exchange of body fluids — it cannot be spread by touching, coughing, or sharing utensils

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Summary

• Measles is spread by airborne droplets, causes a rash and fever, and can be prevented by the MMR vaccine
• HIV is spread by body fluids, attacks the immune system (T helper cells), and is controlled (not cured) by antiretroviral drugs
• TMV is a plant virus that causes a mosaic pattern on leaves, reducing chlorophyll and therefore photosynthesis
• Viruses reproduce by invading host cells and causing lysis — they cannot be treated with antibiotics

Exam Tip: For each named disease you must be able to state the pathogen type, the symptoms, how it is transmitted, and how it is treated or prevented. Practise writing concise answers for each.

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Extended Worked Examples

Worked Example 1 — Measles Outbreak Calculation

A nursery has 120 children. The MMR uptake rate in the local area has fallen from 95% to 82%. Measles needs around 95% vaccine coverage to maintain herd immunity. Estimate how many children are unprotected and comment on the outbreak risk.

Working:

1. Unvaccinated children at 95% coverage: $120 \times 0{.}05 = 6$120×0.05=6.
2. Unvaccinated children at 82% coverage: $120 \times 0{.}18 = 21{.}6 \approx 22$120×0.18=21.6≈22.
3. Increase: $22 - 6 = 16$22−6=16 more susceptible children.