Transmission of Disease
Biology: Cambridge International A Level, topic 10.1.1
Disease Transmission
Diseases that can be passed between animal and plant individuals are known as transmissible or infectious. Transmission of these diseases is either direct or indirect.
Pathogens
- Communicable diseases are caused by harmful microorganisms, called pathogens.
- Examples of pathogenic microorganisms include:
- Bacteria.
- Viruses.
- Protoctista.
- Fungi.
Direct transmission
- Physical contact is made between an infected animal or plant, spreading the pathogen between individuals.
- Direct transmission can take place via bodily fluids such as saliva, blood and semen.
Indirect transmission
- Pathogens are NOT spread through physical contact, but instead by:
- Animal vectors, such as mosquitoes and aphids.
- Air or waterborne transmission by cough droplets or fungal spores.
- Fomites, such as door handles, which are objects onto which pathogens are shed.
- Human movements, which transport infected material on footwear, vehicles and tools.
Non-infectious diseases
- Non-infectious diseases cannot be passed between individuals.
- They can include inherited diseases, such as sickle cell anaemia.
- They also include genetic diseases, like cancer.
Pathogens
- You need to know the following pathogens:
- Vibrio cholerae bacteria, which causes cholera.
- Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium malariae protozoa, which cause malaria.
- Mycobacterium tuberculosis & Mycobacterium bovis bacteria which both cause tuberculosis (TB).
- Human Immunodeficiency Virus (HIV), which causes acquired immunodeficiency syndrome (AIDS).
- Variola virus, which causes smallpox.
- Morbillivirus virus, which causes measles.
Risk Factors for Disease
Not all people are at equal risk of getting communicable diseases. These are some of the factors that can increase an individual's disease risk:
Having a weaker immune system
- A weak immune system makes it much harder to fight off disease.
- People with weak immune systems include:
- Babies and young children.
- Elderly people.
- HIV infection sufferers.
- Those with chronic underlying conditions, like asthma and diabetes.
Living in crowded conditions
- People who live in close proximity to lots of other people are more likely to spread pathogens, especially in:
- Nursing and residential homes.
- Prisons.
- Hospitals.
- Homeless shelters.
Having poor access to healthcare
- Limited access to healthcare or lack of engagement with health professionals can mean diseases are not treated early enough and their progression is more serious.
- Poor healthcare is a key issue in many developing countries.
Test yourself on Transmission of Disease
What are the harmful microorganisms that cause communicable diseases called?
How are pathogens transmitted indirectly?
Which one of these people has the LOWEST risk of disease?
Which groups of people often have weaker immune systems?