Transmission of Disease (Cambridge International AS Biology)

Transmission of Common Diseases

Spec point — 10.1.3 · explain how cholera, malaria, TB and HIV are transmitted

Transmission of cholera

• Cholera is caused by the bacterium **Vibrio cholerae

• The disease is water-borne and food-borne

• Infected people egest large numbers of the pathogenic bacteria in their** faeces **and water becomes contaminated

• Cholera can then be** transmitted when individuals:

  • Wash in contaminated water
  • Drink contaminated water
  • Eat food exposed to contaminated water

• This means the disease occurs where people do not have access to proper sanitation and uncontaminated food

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* as follows:

  • 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
  • When unsterile needles are re-used

• Plasmodium can also pass from mother to child across the placenta

Plasmodium spends part of its life cycle inside humans and part inside mosquitoes

Transmission of tuberculosis (TB)

• TB is cause by the bacterial pathogens Mycobacterium tuberculosis and Mycobacterium bovis

• Human to human transmission occurs by droplet infection as follows:

  • When infected people with the active form of the disease cough or sneeze, M. tuberculosis bacteria enter the air in tiny droplets of liquid
  • TB is transmitted when uninfected people inhale these droplets
  • TB therefore spreads more quickly among people living in overcrowded conditions

• The form of TB caused by M. bovis occurs in cattle but can spread to humans through:

  • Contaminated meat
  • 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 (HIV) can infect human cells and eventually lead to AIDS

• 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
  • Receiving a transfusion of infected blood or blood products
  • 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