Viral Diseases

Viruses are the smallest type of pathogen and cannot reproduce on their own — they must invade a living host cell. This lesson covers the three viral diseases named on the AQA GCSE Biology specification: measles, HIV and tobacco mosaic virus (TMV). You need to know the symptoms, transmission, and treatment or prevention of each.

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How Viruses Cause Disease — A Recap

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

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

This process of cell destruction is what causes the symptoms of viral disease.

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 so damaging and is why the body often cannot fight them quickly.

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Measles

Measles is a highly infectious viral disease that was once extremely common in the UK but is now largely controlled through vaccination.

Feature	Detail
Pathogen	Measles virus (a paramyxovirus)
Transmission	Airborne — spread by droplet infection when an infected person coughs or sneezes
Symptoms	Fever, red skin rash (usually starts on the face and spreads), cough, runny nose, sore eyes
Complications	Can lead to pneumonia and encephalitis (inflammation of the brain), which can be fatal
Prevention	MMR vaccine (measles, mumps, rubella) given in childhood
Treatment	No specific treatment — patients rest and manage symptoms; severe cases may need hospital care

Why Measles Is Dangerous

Measles is one of the most infectious diseases known. If an unvaccinated person is exposed to the measles virus, there is approximately a 90% chance they will become infected. In young children, the elderly, and those with weakened immune systems, measles can lead to life-threatening complications.

The introduction of the MMR vaccine in 1988 dramatically reduced cases in the UK. However, when vaccination rates fall (for example, due to unfounded health scares), measles outbreaks can return.

Exam Tip: If asked about measles, always mention that it spreads by droplet infection (airborne) and that it can be prevented by the MMR vaccine. These are the two key facts examiners look for.

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HIV (Human Immunodeficiency Virus)

HIV is a virus that attacks the body's immune system, specifically the white blood cells (T helper lymphocytes). If untreated, it leads to AIDS (Acquired Immune Deficiency Syndrome).

Feature	Detail
Pathogen	Human Immunodeficiency Virus (HIV)
Transmission	Exchange of body fluids — sexual contact, sharing needles, mother to child during birth or breastfeeding
Initial symptoms	Flu-like illness shortly after infection (fever, sore throat, rash)
Long-term effect	HIV destroys T helper lymphocytes, progressively weakening the immune system
AIDS	When the immune system is severely damaged, the body cannot fight off other infections — these are called opportunistic infections
Prevention	Use of condoms, screening of blood donations, needle exchange programmes, antiretroviral drugs during pregnancy
Treatment	Antiretroviral drugs — these do not cure HIV but control the virus, allowing patients to live long, healthy lives

The Progression from HIV to AIDS

graph TD
    A[HIV infection] --> B[Virus attacks T helper lymphocytes]
    B --> C[Immune system gradually weakens]
    C --> D[Patient becomes vulnerable to opportunistic infections]
    D --> E[Diagnosis of AIDS when immune system is severely compromised]
    F[Antiretroviral therapy] --> G[Virus replication controlled]
    G --> H[Immune system maintained]
    H --> I[AIDS can be prevented]

Important Points About HIV

• HIV is a late-stage disease — a person can carry HIV for many years before symptoms appear
• During this time they can still transmit the virus to others
• Antiretroviral drugs have transformed HIV from a fatal disease into a manageable chronic condition
• There is currently no vaccine and no cure for HIV

Exam Tip: Be careful with the distinction between HIV and AIDS. HIV is the virus; AIDS is the condition that develops when HIV has severely damaged the immune system. They are not the same thing.

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Tobacco Mosaic Virus (TMV)

Tobacco mosaic virus (TMV) is a widespread plant pathogen. It is one of the most well-studied viruses in biology and is named on the AQA specification as an example of a plant disease.

Feature	Detail
Pathogen	Tobacco mosaic virus
Host organisms	Tobacco plants, tomatoes, peppers, and many other species (over 150 plant species)
Transmission	Direct contact between plants, contaminated tools, hands of gardeners, and some insects
Symptoms	Distinctive mosaic pattern of discolouration on leaves — patches of light and dark green or yellow
Effect on plant	Discolouration means less chlorophyll is present, reducing photosynthesis
Consequence	Reduced photosynthesis means the plant cannot grow properly, leading to reduced crop yield
Treatment	No chemical treatment; infected plants should be removed and destroyed to prevent spread
Prevention	Using resistant plant varieties, sterilising tools, good hygiene practices

Why TMV Matters

Although TMV does not infect humans, it is economically important because it damages crop plants. Reduced photosynthesis leads to smaller, weaker plants and lower crop yields, which can cause significant financial losses for farmers.

The mosaic pattern on the leaves occurs because the virus disrupts the production of chloroplasts in the infected cells. Without chloroplasts, those cells cannot photosynthesise, creating the characteristic light-coloured patches.

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