Global burden of disease

• Infectious Agents

  • Viruses
  • Bacteria
  • Fungi
  • Parasites (protozoa and worms)
• Parasitism

  • A type of symbiosis - 'living together'
  • An interaction between two different organisms living in close physical association
• Symbiotic Relationships

  • Mutualistic
  • Commensal
  • Parasitic
• Parasitism (dictionary definition)


  A relationship between two species in which one species benefits and the other species is harmed
• Stages of Infectious Disease

  • Incubation period
  • Prodromal period
  • Illness
  • Convalescence
• Biological Response Gradient

  Clinical picture depends on: infecting dose, age of host, biological sex of host, host genetics, host nutritional status, co-infection with other pathogens
• Frequency of clinically apparent disease

  • EBV (child) - 1.0%
  • Influenza (young adult) - 60%
  • Rabies - 100%
• Infectious Disease Stages

  • Invasion
  • Multiplication
  • Spread
  • Pathogenesis
• Invasion - Transmission Routes

  • Oral
  • Intra-uterine
  • Direct skin contact
  • Inhalation
  • Sexual
  • Direct inoculation (injections, insect bites)
• Multiplication

  • Protists can multiply within the body, disease severity depends on multiplication rate
  • Helminths cannot multiply within the body, disease severity depends on number of infectious stages acquired
• Spread

  • Ability of organism to move from initial site of infection to infect other areas of the body
  • Some infectious agents undergo developmental changes which can have implications for host immune responses
• Pathogenesis

  Causation and development of clinical disease, influenced by: number of pathogenic organisms, virulence of organism, reaction of the host
• Incidence

  Number of new cases of infection occurring in a population in a defined period of time
• Prevalence


  Total number of infected individuals in a population at a given point in time
• Incidence vs Prevalence

  Incidence measures new cases, prevalence measures total cases
• In some countries there has been a shift in the burden of infectious diseases
• Leading Causes of Death in USA, 1900 and 1997

  • 1900: Pneumonia, Tuberculosis, Diarrhea & enteritis

  1997: Heart disease, Chronic lung disease, Injuries, Pneumonia & influenza, Diabetes, HIV, Suicide, Liver disease, Cancer, Stroke
• In 1900, 30.4% of all deaths occurred among children aged <5 years; in 1997, that percentage was 1.4%
• In other countries and for low income groups, infectious diseases still have major impact on health
• Major differences exist between 'rich' and 'poor' countries with respect to causes of death
• Causes of Death - High-income vs Low-income Countries

  • High-income: Chronic diseases (cardiovascular, cancer, dementia, lung disease, diabetes)

  Low-income: Infectious diseases (respiratory, HIV/AIDS, diarrheal, malaria, TB)
• In low-income countries, infectious diseases collectively account for 1/3 of deaths
• Mortality does not give complete picture of burden of disease borne by individuals in different populations
• Disability Adjusted Life Year (DALY)


  Measures 'life years' lost due to premature mortality and equivalent years lost because of morbidity (lower 'quality of life')

  Calculated as: Years of Life Lost (YLL) + Years Lost to Disability (YLD)

  Allows comparisons across health problems, quantitative basis for health policies and cost-effectiveness
• DALYs only measure direct health loss and do not consider economic impacts, direct costs of treatment/prevention, or social stigma
• Factors contributing to increasing importance of infectious diseases

  • Drug resistant pathogens and vectors

  Movement of refugees

  Rapid and widespread air travel

  Increase in immuno-deficient people

  Lifestyle factors (urbanisation, IV drug use, sexual behaviour)

  Natural/social disasters, poverty, deprivation

  Environmental changes (global warming, climate change, deforestation, irrigation, antimicrobial use)

  Pandemics (COVID-19, influenza)
• COVID-19 vaccine inequality exists between countries
• incubation period

  time between infection and the occurrence of first symptoms or signs of disease
• prodromal period is the period of time before the onset of symptoms
• illness: most severe stage when symptoms are most evident and host immune system not fully responded
• Convalescence: body gradually returns to normal (time varies based on pathogen)
• mortality: total number of death from disease in a population in a defined period of time
• 19th centry = overcrowding and poor housing
• What is used to measure overall burden of disease?

  DALY - disability adjusted life year