Transmission of Disease (CIE AS Biology)

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 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

The life cycle of Plasmodium

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