CIE A Level Biology (9700) 2019-2021

Revision Notes

10.1.4 Global Patterns in Disease

Factors that Influence Global Patterns of Distribution of Common Diseases

  • Malaria, tuberculosis (TB) and HIV/AIDS are three infectious diseases of worldwide importance
  • The number of people infected with these diseases remains very high and these diseases pose serious public health problems now and for the foreseeable future (particularly in parts of the world that lack the developed and efficient health services of high-income countries)

Factors affecting the distribution of malaria

  • Malaria is caused by one of four species of the protoctist Plasmodium but these protoctists are transmitted to humans by an insect vector (female Anopheles mosquitoes)
  • The Anopheles mosquitoes favour habitats that have high rainfall, high temperatures and high humidity
  • This means malaria can occur where these mosquitoes are present and, as a result, is found throughout the tropics and sub-tropics (about 80% of cases are in Africa)
  • The Anopheles mosquitoes found in Africa also have longer lifespans and prefer biting humans than animals
  • In the 1950s, the World Health Organisation (WHO) coordinated a worldwide eradication programme. Whilst malaria was eradicated from some countries, the programme was mainly unsuccessful because:
    • Plasmodium became resistant to the drugs being used to try and control it
    • Anopheles mosquitoes became resistant to DDT and other insecticides being used against them
  • There is evidence that there are an increasing number of malaria epidemics due to climatic and environmental changes that favour the spread of the Anopheles mosquitoes
  • A social factor that has caused the number of cases of malaria in Africa, in particular, to increase in recent years is the increased migration of people due to war (when migration happens due to war the parasite can be transferred from areas that have the infection to new regions, and, if the Anopheles mosquito is breeding in the new region, then the mosquito vector will transfer the disease from one human to the next)

Factors affecting the distribution of tuberculosis (TB)

  • TB has a worldwide global distribution
  • TB is spread quickly from person to person when droplets released by the coughing or sneezing of an infected person with the active form of the illness are inhaled by an uninfected person (the droplets contain the TB-causing bacterium Mycobacterium tuberculosis)
  • TB, therefore, spreads more quickly among people living in overcrowded conditions
  • People who sleep close together in large numbers are at a higher risk
  • TB primarily affects the homeless and people who live in poor, substandard housing
  • Some people may be infected with TB but not show symptoms. Whilst the bacteria is inactive there is no spread of the disease however if the person becomes infected with HIV (TB can be the first opportunistic infection for HIV-positive people) or is malnourished they can become infectious thus contributing to the spread
  • High rates of TB can also be found in cities with populations of migrants from countries where TB is more common (eg. parts of London have much higher TB rates than the rest of the UK)

Factors affecting the distribution of HIV/AIDS

  • HIV/AIDS has a worldwide global distribution but is especially prevalent in sub-Saharan Africa and South-East Asia, mainly due to a lack of education about how the virus is transmitted and a lack of access to items that offer protection from the virus during sexual intercourse such as condoms and femidoms
  • 70% of the world’s deaths from AIDS occur in Africa
  • AIDS is widespread throughout sub-Sahara Africa and is a serious public health problem because HIV infection makes people more vulnerable to existing diseases such as malnutrition, TB and malaria, which are all prevalent there

Annual worldwide incidence & mortality rates table

Annual worldwide incidence and mortality rates table, downloadable AS & A Level Biology revision notes

Exam Tip

You do not need to learn these figures for annual worldwide incidence and mortality rates for malaria, TB and HIV/AIDS but make sure you learn where these three infectious diseases are distributed around the globe and some of the main factors that influence the distribution patterns of each disease.


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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