disease

• Disease


  An illness or disorder of the body or mind that leads to poor health
• Communicable/infectious disease


  Disease caused by pathogens and is transmissible (can be spread between individuals within a population)
• Infectious disease


  Disease caused by a pathogen that passes from infected individuals to uninfected individuals
• Infectious diseases


  • Cholera
  • HIV/AIDS
  • Malaria
  • Tuberculosis
• Non-infectious disease


  Long-term diseases that are not caused by pathogens
• Pathogens


  • Bacteria
  • Viruses
  • Fungi
  • Protoctists
• Bacteria


  Diverse range of prokaryotic organisms, some are pathogenic and some are non-pathogenic. Cause damage by infecting host cells and release toxins causing damage to the cells and tissue
• Pathogenic bacteria


  • M. tuberculosis (causes tuberculosis)
  • N. meningitidis (causes bacterial meningitis)
• Viruses


  Do not have a cellular structure, they infect host cells and hijack their machinery to replicate
• Viruses


  • Tobacco Mosaic Virus (TMV)
  • Influenza A, B, C (cause the flu)
  • Human Immunodeficiency Virus (HIV)
• Protoctista


  Unicellular eukaryotes
• Protoctist pathogens


  • Plasmodium falciparum (causes malaria)
  • P. infestans (causes potato blight)

  
• Fungi


  Eukaryotic organisms with cell walls and large central vacuoles, their bodies consist of filaments called hyphae
• Direct transmission


  Transfer of pathogens from an infected host to an uninfected host through physical contact or close proximity
• Direct transmission


  • Leaf-to-leaf contact of plants infected with Tobacco Mosaic Virus
  • Airborne droplets containing influenza viruses
  • Spores dispersed by wind or water
• Indirect transmission


  Transfer of pathogens from an infected host to an uninfected host via a second organism (vector) that is unaffected by the pathogen
• Indirect transmission


  • Female Anopheles mosquitoes transmitting Plasmodium (malaria)

  
• Factors affecting disease transmission

  • Presence of pathogens
  • Presence of susceptible individuals
  • Proportion of resistant or immune individuals in the population
• Factors affecting direct transmission

  • Population density (e.g. cities, schools)
  • Monocultures in agriculture
• Factors affecting indirect transmission

  • Biology of the vector (e.g. mosquitoes, aphids)
  • Weather and climate
• Infection


  Need potential hosts to be within close proximity to each other
• Places/areas with high population densities

  • More likely to have high infection rates
  • E.g. cities and schools
• Tuberculosis (TB) transmission


  • Very high in places where many people have to sleep in confined quarters
  • E.g. poor housing and homeless shelters
• Farmers use monocultures to maximise yield and profit


  Can experience large disease outbreaks
• Farmers grow a large number of crop plants in a small area

  As the crops grow the leaves of different plants touch each other, making the transmission of pathogens such as tobacco mosaic virus (TMV) very easy
• Indirect transmission


  The indirect transmission of a pathogen can be affected by the biology of the vectors involved
• Common disease vectors


  • Mosquitoes and aphids

  
• Vector population


  Influenced by weather and climate
• Malaria


  • Caused by one of four species of the protoctist Plasmodium
  • Transmitted to humans by an insect vector (female Anopheles mosquitoes)

  
• Anopheles mosquitoes


  • Favour habitats that have high rainfall, high temperatures and high humidity
  • Found throughout the tropics and sub-tropics (about 80% of cases are 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, but it was mainly unsuccessful
• Reasons for the failure of the WHO eradication programme

  • 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

  
• Increased migration of people due to war

  Can transfer the malaria parasite 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 transmission of human diseases

  • The degree or level of poverty in an area often correlates with the transmission of human diseases in that area
  • Water-borne diseases like typhoid, cholera and polio spread when human faecal matter enters and contaminates drinking water
  • Those below the poverty line usually live in areas with crowded housing with no sewage systems, sanitation facilities or water treatment facilities, and have limited access to hygiene products
• In the last 200 years, humans have spread across the globe, bringing their diseases and pathogens with them
• The level of human movement and migration that currently exists means that populations are more connected than ever
• In the past, the ocean and bodies of water would have acted as natural geographic barriers to prevent the spread of pathogens


  The first flu pandemic in 1918 took one year to spread around the globe, while the flu pandemic in 2009 only took 3 months to reach West Africa from North America
• An individual can become infected in one country (where the disease exists) and get a flight to another country thousands of miles away, and may not show any symptoms until they have already arrived in the new country
• When colonisers arrived in the Americas they brought many European diseases with them, such as smallpox, and the Native Americans had no immunity or resistance as they had never been exposed to these pathogens before, causing a large number of deaths