CIE AS Biology (9700) 2019-2021

Revision Notes

10.1.2 Transmission of Disease

Transmission of Common Diseases

Transmission of cholera

  • Cholera is caused by the bacterium Vibrio cholerae
  • The disease is water-borne (the bacterium lives in water) and food-borne
  • This means the disease occurs where people do not have access to proper sanitation (clean water supply) and uncontaminated food
  • Cholera can be transmitted when people; bath or wash in contaminated water, drink contaminated water or eat food exposed to contaminated water
  • Infected people egest large numbers of the bacteria in their faeces
  • If these faeces contaminate the water supply, or if infected people handle food or cooking utensils without washing their hands, then the bacteria are transmitted to uninfected people

Transmission of measles

  • Measles is caused by a highly infectious virus Measles morbillivirus
  • The virus is transmitted via the millions of tiny droplets discharged from the nose and mouth of an infected person when they cough or sneeze
  • Measles is therefore caught when someone breathes in these droplets or touches a surface contaminated by them (the virus can survive several hours on a surface)
  • Infected individuals are contagious from the onset of symptoms until about 4 days after the rash first appears

Transmission of malaria

  • Malaria is caused by one of four species of the protoctist Plasmodium
  • These protoctists are transmitted to humans by an insect vector:
    • Female Anopheles mosquitoes feed on human blood to obtain the protein they need to develop their eggs
    • If the person they bite is infected with Plasmodium, the mosquito will take up some of the pathogen with the blood meal
    • When feeding on the next human, Plasmodium pass from the mosquito to the new human’s blood
  • Malaria may also be transmitted during blood transfusion and when unsterile needles are re-used
  • Plasmodium can also pass from mother to child across the placenta

Transmission of tuberculosis (TB)

  • When infected people with the active form of the disease cough or sneeze, the Mycobacterium tuberculosis bacteria enter the air in tiny droplets of liquid
  • TB is transmitted when uninfected people then inhale these droplets
  • TB therefore spreads more quickly among people living in overcrowded conditions
  • The form of TB caused by Mycobacterium bovis occurs in cattle but is spread to humans through contaminated meat and unpasteurised milk
  • Very few people in developed countries now acquire TB in this way, although meat and milk can still be a source of infection in some developing countries

Transmission of HIV/AIDS

  • Human Immunodeficiency Virus is a retrovirus
  • The HIV virus is not transmitted by a vector (unlike in malaria)
  • The virus is unable to survive outside of the human body
  • The virus is spread by intimate human contact and can only be transmitted by direct exchange of body fluids
  • This means HIV can be transmitted in the following ways:
    • sexual intercourse
    • blood donation
    • sharing of needles used by intravenous drug users
    • from mother to child across the placenta
    • mixing of blood between mother and child during birth
    • from mother to child through breast milk

Cholera, malaria, TB & HIV/AIDS summary table

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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