Global impact of infectious diseases

Types of Symbiosis

Symbiosis = Interaction between two species living in close association

• Parasitism: One species benefits, the other is harmed
  (e.g. Helminths, Protists)

Major Classes of Infectious Agents

Chronic - Long-term, persistent infection

Protists - Unicellular pathogens (e.g. malaria)

Helminths - Multicellular worms (e.g. schistosomiasis)

Clinical threshold

When symptoms first become noticeable

Stages of infectious disease

1) Incubation – Time from infection to first symptoms

2) Prodromal – Mild, general symptoms (fatigue, etc.)

3) Illness – Most severe symptoms, immune system not fully active

4) Convalescence – Gradual return to normal

Infectious Process

1) Invasion

2) Multiplication

3) Spread

4) Pathogenesis

Invasion

Pathogen enters host & transmits

Multiplication

• Protists: Multiply inside host

• Helminths: Do not multiply inside host

Spread

Infection moves to other body areas

Pathogenesis

1. Disease development

Pathogenesis Depends on:

• Number of pathogens

• Virulence factors:

  • Direct cell killing (e.g., Plasmodium)

  • Blockages (e.g., Plasmodium)

  • Toxins (e.g., Yersinia pestis)

  • Immune system overreaction (e.g., schistosomes)

• Host’s immune resistance

• Movement across body systems (e.g. nervous, circulatory)

Measuring disease burden

Incidence - New cases in a time period

Prevalence - Total cases at a point in time

Mortality - Total deaths in a time period

DALY – Disability Adjusted Life Years

DALY = YLL (Years of Life Lost) + YLD (Years Lived with Disability)

Used to: Compare disease burden across populations and prioritize health policies
Limitations:

• Doesn’t measure economic impact or healthcare cost

• Ignores social stigma, school/work impact, healthcare system strain