Transmission of Disease (CIE A Level Biology)

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, or 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 and 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 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 (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
  • 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

Disease	Pathogen	Transmission	Symptoms
Cholera	Vibrio cholerae	Contaminated water or food	Diarrhoea and dehydration
Malaria	Plasmodium falciparum, P. malariae, P. ovale, P. vivax	Female Anopheles mosquito	Fever, headaches and muscle pain
TB	Mycobacterium tuberlulosis, M. bovis	Airborne droplets	Cough, chest pain, fever and weight loss
HIV/AIDS	Human Immunodeficiency Virus	Exchange of body fluids	Initial flu-like symptoms, and eventually a loss of immune function (AIDS)