Viral diseases (AQA GCSE Biology Combined Science): Revision Notes
Viral diseases
What are viruses?
Viruses are a type of pathogen. Pathogens are microorganisms that cause infectious diseases in living things.
A virus is made up of genetic material (DNA or RNA) wrapped in a coat made of proteins and lipids. This makes them much simpler than normal cells.
Unlike bacteria or other microorganisms, viruses are not technically alive because they cannot carry out life processes on their own. They exist in a state between living and non-living matter.
Key features of viruses
Viruses have several important characteristics that you need to remember:
- They are much smaller than cells
- They can only reproduce inside living cells
- They cause damage to the cells they infect
- They reproduce very quickly once inside the body
- They can infect both plants and animals
- They spread through direct contact, water or air
Common Example: Respiratory Viruses
Colds and flu are both caused by viruses. When someone with a cold sneezes, millions of viral particles are released into the air, which can then be breathed in by others nearby.
How viruses reproduce - the virus life cycle
Viruses cannot reproduce on their own. They need to hijack living cells to make copies of themselves. This parasitic relationship is what makes viral infections so damaging to the host organism.
The Virus Life Cycle: Step-by-Step Process
- Entry: The virus enters a cell through the cell membrane
- Hijacking: The cell starts making viral DNA/RNA and viral proteins instead of its normal work
- Assembly: New viruses are built inside the cell
- Release: New viruses leave the cell (often bursting it open) and go on to infect other cells
This process damages or destroys the host cells, which is why we feel ill when infected with viruses. The symptoms we experience are often the result of our immune system fighting the infection and the damage caused by viral reproduction.
Human viral diseases
Measles virus
How it spreads:
- Through tiny droplets when infected people sneeze or cough
- These droplets can be breathed in by other people
Symptoms:
- High fever
- Red skin rash all over the body
- Can be fatalif serious complications develop
Prevention: Most young children are vaccinated against measles to protect them.
Measles is highly contagious - one infected person can spread it to 12-18 other people in a population that isn't vaccinated. This is why vaccination programmes are so crucial for public health.
HIV (Human Immunodeficiency Virus)
How it spreads:
- Sexual contact between infected and uninfected people
- Exchange of body fluids like blood (for example, when drug users share needles)
What happens in the body:
- Causes flu-like symptoms at first
- Attacks the body's immune cells - the cells that normally fight infections
- Without treatment with special drugs, it develops into AIDS
- AIDS happens when the immune system is too damaged to fight other infections or cancers
HIV specifically targets CD4+ T cells, which are crucial components of the immune system. As these cells are destroyed, the body becomes increasingly vulnerable to opportunistic infections and certain cancers.
Plant viral diseases
Tobacco mosaic virus (TMV)
TMV doesn't just affect tobacco plants - it can infect many different plant species, making it a significant concern for agriculture and gardening.
Symptoms: The leaves develop a distinctive mottled or 'mosaic' pattern of light and dark patches. This damage reduces photosynthesis, so the plant grows more slowly.
How it spreads: The virus can move from cell to cell within the plant, and from plant to plant through direct contact.
TMV is extremely stable and can survive outside of living cells for extended periods. It can even survive on tools, clothing, or hands, which is why gardeners are advised to wash their hands and sterilise tools when working with potentially infected plants.
Key Points to Remember:
- Viruses are pathogens made of genetic material wrapped in protein and lipid coats
- They can only reproduce inside living cells, causing damage in the process
- They reproduce very rapidly once inside the body
- Measles spreads through droplets and causes fever and rashes - vaccination protects against it
- HIV spreads through body fluids and attacks immune cells, leading to AIDS without treatment
- Plant viruses like TMV create distinctive patterns on leaves and slow down plant growth