Transmission

• Disease transmission


  The transfer of pathogens from an infected host to an uninfected host
• If pathogens are unable to find new hosts then they will go extinct
• Types of disease transmission

  • Direct from one host to another host
  • Indirect via a second organism (vector) that is unaffected by the pathogen
• Direct transmission


  1. Physical contact between individuals

  2. Individuals being within close proximity to each other

  3. Spores dispersed via wind or water
• Spores


  Very small reproductive structures that are released into the environment, can be haploid or diploid
• Transmission of HIV/AIDS


  Spread by intimate human contact and can only be transmitted by direct exchange of body fluids
• Transmission of tuberculosis (TB)

  1. Infected people cough or sneeze, Mycobacterium tuberculosis bacteria enter the air in tiny droplets

  2. Uninfected people inhale these droplets
• Indirect transmission


  Vectors are involved, organisms that transfer a pathogen from an infected individual to an uninfected individual
• Disease vectors


  • Insects
• Transmission of malaria


  1. Female Anopheles mosquitoes feed on human blood

  2. If the person they bite is infected with Plasmodium, the mosquito will take up some of the pathogen

  3. 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
• Factors affecting disease transmission

  • Presence of pathogens
  • Presence of susceptible individuals
• Resistance


  Mechanisms that prevent the infection or spread of pathogens within the body, coded for by genes
• Immunity


  Previously infected with the pathogen, suffered symptoms and recovered, highly unlikely to develop symptoms again
• The higher the proportion of resistant or immune individuals, the lower the probability of transmission
• Factors affecting direct transmission

  • Need potential hosts to be within close proximity
  • High population densities more likely to have high infection rates
  • Monocultures can experience large disease outbreaks
• Factors affecting indirect transmission

  • Biology of the vectors involved
  • Population of vectors influenced by weather and climate
• Malaria is found throughout the tropics and sub-tropics (about 80% of cases are in Africa)
• Malaria eradication programme was mainly unsuccessful due to Plasmodium and Anopheles mosquitoes becoming resistant
• Increasing number of malaria epidemics due to climatic and environmental changes that favour the spread of Anopheles mosquitoes
• Increased migration of people due to war has caused the number of malaria cases in Africa to increase
• Factors affecting transmission of human diseases

  • Degree of poverty correlates with transmission
  • Water-borne diseases spread when human faecal matter contaminates drinking water
  • High level of human movement and migration means populations are more connected than ever
• When colonisers arrived in the Americas they brought many European diseases with them, such as smallpox, which caused large numbers of deaths in the Native American population
• Endemic


  A disease that is always present in a population (even if very low numbers)
• Epidemic


  A large increase in the number of cases in a population (an outbreak)
• Pandemic


  An epidemic that occurs on a large scale and crosses international boundaries