Viral diseases (AQA GCSE Biology): Revision Notes
Viral diseases
What are viruses?
Viruses are tiny pathogens that cause infectious diseases. A pathogen is any microorganism that can make you ill.
Understanding the basic structure of viruses helps explain how they cause disease and why they're so effective at spreading.
Structure of a virus:
- Contains genetic material (either DNA or RNA)
- Surrounded by a protective coat made of proteins and lipids
- Much smaller than normal cells
Key features of viruses
Viruses have several important characteristics that make them different from other pathogens. These unique features explain why viral infections can be so challenging to treat and why they spread so effectively.
- Size: They are much smaller than typical cells
- Reproduction: They can only reproduce inside living cells (called host cells)
- Speed: They multiply very quickly once inside the body
- Damage: They harm cells as they reproduce, causing illness
- Hosts: They can infect both plants and animals
- Spread: They pass from person to person through direct contact, droplets in air, or contaminated water
Examples of common viral diseases include colds and flu.
How viruses reproduce
Viruses follow a specific life cycle when they infect cells. This process is crucial to understanding how viral infections develop and why they cause illness.
Worked Example: The Viral Life Cycle
Step 1: Entry - The virus gets into a cell through the cell membrane
Step 2: Production - The infected cell starts making more viral DNA/RNA and viral proteins
Step 3: Assembly - New virus particles are built inside the cell
Step 4: Release - The new viruses either leave the cell or cause it to burst open
This process damages or destroys the host cell, which is why viral infections make us feel unwell. The rapid multiplication of viruses means that a single infected cell can produce hundreds or thousands of new viral particles.
Human viral diseases
Measles virus
The measles virus is highly contagious and can cause serious complications, especially in young children and adults with weakened immune systems.
How it spreads:
- Through tiny droplets when infected people sneeze or cough
- These droplets can be breathed in by others
Symptoms:
- High temperature (fever)
- Red skin rash all over the body
- Can be very dangerous if complications develop
- Most children are now vaccinated against measles to prevent infection
Measles can lead to serious complications including brain inflammation and pneumonia, which is why vaccination is so important for public health.
HIV (Human Immunodeficiency Virus)
HIV is a particularly serious viral infection because it attacks the body's immune system, making patients vulnerable to other infections and diseases.
How it spreads:
- Through sexual contact with infected people
- By sharing needles (common among drug users)
- Through contact with infected blood or other body fluids
What happens in the body:
- Starts with flu-like symptoms
- Attacks the body's immune system cells
- Without treatment with special antiviral drugs, it gets worse
- Can develop into AIDS when the immune system becomes too weak to fight other infections or cancers
Early diagnosis and treatment with antiviral drugs can help people with HIV live normal, healthy lives and prevent transmission to others.
Plant viral diseases
Plants can also be infected by viruses, which can significantly impact crop yields and plant health in both agricultural and natural ecosystems.
Tobacco Mosaic Virus (TMV)
TMV doesn't just affect tobacco plants - it can infect tomatoes and many other plant species too.
Symptoms in plants:
- Leaves develop a distinctive mottled or 'mosaic' pattern
- Leaves become discoloured
- This reduces photosynthesis, so the plant grows more slowly
How it spreads:
- Direct contact between plants
- Can move from cell to cell within the plant
- Can also travel through the plant's transport system (phloem)
The reduction in photosynthesis caused by TMV can significantly impact plant growth and crop yields, making it an important concern for farmers and gardeners.
Key Points to Remember:
- Viruses are pathogens - they cause infectious diseases by reproducing inside host cells
- They're very small - much smaller than bacteria or our own cells
- Common human viral diseases include measles (spread by coughing/sneezing) and HIV (spread through body fluids)
- Viruses damage cells as they reproduce, which causes the symptoms of illness
- They affect plants too - TMV causes mosaic patterns on leaves and slows plant growth