Viral Diseases
For Edexcel GCSE Biology you need to know the details of three specific viral diseases: HIV, tobacco mosaic virus (TMV), and measles. You also need to understand why viral diseases are difficult to treat.
How Viruses Cause Disease — A Recap
Viruses are not true living cells. They are tiny particles consisting of a piece of genetic material (DNA or RNA) surrounded by a protein coat (capsid). Some viruses also have a lipid envelope.
How they work:
- The virus attaches to a specific receptor on the host cell surface
- It injects its genetic material (or enters the cell whole)
- The viral genetic material takes over the cell's machinery
- The cell is forced to produce many copies of the virus
- The host cell fills with new virus particles and eventually bursts (lysis)
- New viruses are released to infect neighbouring cells
This destruction of host cells causes the symptoms of viral disease.
1. HIV (Human Immunodeficiency Virus)
What It Is
HIV is a virus that attacks the immune system. Specifically, it infects and destroys T helper lymphocytes (also called CD4 cells) — a type of white blood cell that plays a central role in coordinating the immune response.
How It Is Spread
- Sexual contact (unprotected sex) — the most common route of transmission
- Sharing contaminated needles (e.g. among intravenous drug users)
- Exchange of body fluids containing the virus (blood, semen, vaginal fluids, breast milk)
- Mother to child during pregnancy, birth, or breastfeeding (though this risk can be greatly reduced with treatment)
HIV is NOT spread by casual contact, coughing, sneezing, sharing cutlery, or mosquito bites.
Stages of HIV Infection
| Stage | What happens |
|---|
| Initial infection | Flu-like symptoms for a few weeks (fever, sore throat, rash). The person is highly infectious. |
| Latent phase | The virus continues to replicate and destroy T helper cells, but the person may have no symptoms for years. They can still transmit the virus. |
| AIDS | When the T helper cell count drops critically low, the immune system can no longer fight infections. The person develops Acquired Immune Deficiency Syndrome (AIDS). They become vulnerable to opportunistic infections (e.g. TB, pneumonia) and certain cancers. |
Treatment and Prevention
- No cure exists for HIV
- Antiretroviral drugs (ARVs) can control the virus by slowing its replication, keeping the viral load low and the immune system functioning — people on effective treatment can live long, healthy lives
- ARVs must be taken daily for life
- Prevention: use of condoms (barrier contraception), not sharing needles, screening blood donations, antiretroviral treatment during pregnancy to prevent mother-to-child transmission
Exam tip: HIV does not directly cause death. It weakens the immune system so that the person dies from opportunistic infections that a healthy immune system would normally fight off. Make sure you explain this clearly in extended-response questions.
2. Tobacco Mosaic Virus (TMV)
What It Is
TMV is a virus that infects plants, particularly tobacco plants, tomatoes, peppers, and other members of the Solanaceae family. It is one of the most studied viruses in biology.
Symptoms
- Mosaic pattern on the leaves — patches of discolouration (light and dark green, yellow, or white) in an irregular mosaic pattern
- Stunted growth of the plant
- Reduced photosynthesis — the discoloured parts of the leaves contain less chlorophyll, so the plant cannot photosynthesise as effectively, reducing growth and crop yield
How It Spreads
- Direct contact between plants
- Through contaminated tools, hands, or soil
- TMV is extremely stable and can survive outside a host for long periods on surfaces
- It can also spread through seeds from infected plants
Impact
- TMV is a major agricultural problem because it reduces crop yields
- There is no chemical treatment for TMV once a plant is infected
- Prevention involves removing and destroying infected plants, sterilising tools, and growing resistant plant varieties
Exam tip: TMV is a plant disease. Do not confuse it with a human disease. The key symptom is the mosaic pattern on the leaves and the consequence is reduced photosynthesis because of less chlorophyll in the discoloured areas.
3. Measles
What It Is
Measles is a highly contagious viral disease caused by the measles virus (a paramyxovirus). It primarily affects children, though unvaccinated adults can also be infected.
Symptoms
- High fever
- Red-brown skin rash — typically starts on the face and spreads to the body
- Cough, runny nose, and sore eyes (conjunctivitis)
- Small white spots inside the mouth (Koplik's spots) — an early diagnostic sign
Complications
Measles can lead to serious, potentially fatal complications, especially in young children, the malnourished, or those with weakened immune systems:
- Pneumonia (lung infection)
- Encephalitis (swelling of the brain) — can cause brain damage
- Death — measles kills over 100,000 people worldwide each year, mostly children in developing countries
How It Spreads
- Airborne droplet infection — spread when an infected person coughs or sneezes
- Measles is extremely contagious — one infected person can infect 12–18 unvaccinated people
Treatment and Prevention
- No specific cure — treatment is supportive (rest, fluids, paracetamol for fever)
- Prevention: the MMR vaccine (measles, mumps, rubella) is highly effective
- Most children in the UK receive two doses of MMR — at 12–13 months and at around 3 years 4 months
- The MMR vaccine has dramatically reduced measles cases in countries with high vaccination rates
Exam tip: Measles can be fatal, especially in young children. Always mention this if asked about the severity of measles. Also, remember that measles is prevented by vaccination, not by antibiotics (antibiotics do not work on viruses).
Why Are Viral Diseases Difficult to Treat?
This is a crucial concept: