Transmission and Infectivity

Introduction by Silvana Guderian on transmission routes, infectivity, and virulence of infectious diseases.
Historical context: Pathogens have significantly impacted human populations through outbreaks and pandemics.

Bubonic Plague (1300s): Estimated to have killed about 200 million people, affecting various regions globally.
Spanish Flu (1918-1919): Estimated to have resulted in an extremely high mortality rate during its outbreak.
HIV/AIDS: Since the 1980s, approximately 25 to 35 million people are estimated to have died from HIV-related causes globally.
COVID-19 (Coronavirus outbreak): Initial data indicated a small number of deaths, but recent figures show millions affected globally.

WHO Report (2019): Listed current global health issues, emphasizing infectious pathogens (in red) and related issues like antimicrobial resistance and vaccine hesitancy.
Continued influence of infectious diseases: Despite improvements in sanitation and medicine, these diseases remain a significant health threat.

Population Density: Higher density increases the risk of exposure to respiratory infections (e.g., influenza, measles).
Contaminated Water Sources: Important in fecal-oral diseases (e.g., cholera, typhoid).
Environmental Factors: Weather conditions can affect vector presence and activity, influencing disease transmission.
Agricultural Practices: Changes from hunter-gatherer to agricultural societies have increased contact with vectors and pathogens.

Hunter-Gatherer vs. Agricultural Societies:
- Hunter-gatherers have smaller, mobile populations, while agricultural populations have denser groups capable of supporting larger vector populations.
- Trade and migration have historically influenced pathogen spread, showing that contact with naive populations can lead to severe outbreaks.
- Improvements in sanitation have led to a decreased risk of exposure to some diseases but can be compromised by infrastructure breakdowns.

Malaria is transmitted by the female Anopheles mosquito, resulting in high morbidity and mortality rates, especially in children.
Endemic Areas: Distinct geographic areas show varying prevalence rates of malaria, influenced by genetic factors as well.
Genetic variations, like the sickle cell trait, provide some individuals with protection against severe malaria.

Definition: Represents the average number of new infections caused by an infected individual in a completely susceptible population.
- Calculating R0:
- Numerator: $b imes N$ (where $b$ is the probability of infection and $N$ is the number of susceptible individuals).
- Denominator: $D + V$ ( $D$ is mortality unrelated to the pathogen, $V$ is the recovery rate).
Virulence: Measured through the case fatality ratio, contributing to calculations of R0.
Impact on Herd Immunity: Determines vaccination thresholds to prevent outbreaks.

Infectiousness vs Symptoms: Understanding when individuals are contagious is crucial for outbreak prevention and control.
Case studies: SARS 2003, pandemic influenza, and HIV show varying timelines of symptom onset and infectiousness.

Various stages of disease can provide insights into transmissibility and public health planning:
- Stage I: Low human-to-human transmission (e.g., rabies).
- Stage II: Limited cycles of transmission (e.g., Ebola).
- Stage III and IV: Significant potential for global outbreaks (e.g., pandemic influenza).
- Examples of antigenic shifts in influenza, leading to pandemics (e.g., Spanish Flu, 2009 H1N1).

Life Expectancy: The Spanish flu drastically lowered life expectancy in the U.S. in 1918.
Comparing Infectious vs Non-communicable Diseases: Patterns differ by income level and regional health practices (greater infectious diseases in low-income countries, non-communicable diseases in high-income countries).

Highlighted infectious diseases impacting life expectancy and health globally. COVID-19 additions are necessary for recent discussions on global health.