Global impact

• 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
• Infectious Disease Stages


  • Invasion
  • Multiplication
  • Spread
  • Pathogenesis
• Invasion - Transmission routes


  • Oral
  • Direct skin contact
  • Inhalation
  • Sexual
  • Direct inoculation
  • Insect bites - vector-borne
• 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 other countries and for low income groups, infectious diseases still have major impact on health
• Differences between High- and Low-income countries


  • High-income: 70% deaths in 70+ years, 1% deaths in <15 years, predominantly chronic diseases
  • Low-income: 40% deaths in <15 years, 20% deaths in 70+ years, predominantly infectious diseases
• 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 by adding 'years of life lost (YLL) to premature mortality' and 'years lost to (lived with) disability (YLD)'
  • Allows comparisons across health problems, quantitative basis for health policies and cost-effectiveness of control programmes
• DALYs only measure direct health loss and do not consider economic impacts, direct costs of treatment, surveillance, 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)