Epidemiology: Tracking Infectious Diseases

Epidemiology

Scientific study of how diseases are distributed in populations, and the factors that influence or determine that distribution

Epidemiologic Triad

the traditional model for infectious disease, consisting of an agent, host, and an environment

Chain of Infection

A sequence of events required for a disease to spread

reservoir, portal of exit, mode of transmission, susceptible host, portal of entry

An infectious agent leaves its_______ through a _______, is transmitted via a ________, and enters a ________ through a ________.

Incidence

The rate at which new cases occur in a population over a specific time

Prevalence

the total number of existing cases in a population at a given point in time

Infectivity

the ability of an agent to enter and multiply in a host

Pathogenicity

the ability of an agent to cause clinical disease after infection

Virulence

the degree of severity of the disease produced

Descriptive Epidemiology

Organizes data by person, place, and time to find patterns

Analytic Epidemiology

uses comparison groups to test hypotheses about the link between exposure and disease, often through cohort studies or case control studies

Surveillance Systems

Continuous, systematic collection and analysis of health data to monitor disease trends and detect outbreaks early.

Reproductive Number (R_t or R₀)

A metric estimating the average number of new infections caused by one infected person.

R_t  above 1.0 

indicates an increasing epidemic 

R_t  below 1.0 

declining epidemic

source, transmission, trends

Epidemiology illustrates the movement of disease by systematically investigating three key areas:

Disease Emergence

New infectious diseases emerge when pathogens cross from animals to humans or adapt to new environments

zoonotic

Most emerging infectious diseases (EIDs) are _______, meaning they originate in animals before jumping to humans.

Zoonotic Spillover

Examples include HIV, which likely crossed from chimpanzees to humans via bushmeat hunting, and SARS, which emerged from contact with wild animals like civets.

Ecological Changes

Human activities like deforestation and urbanization bring people closer to wildlife. For example, reforestation in the U.S. increased deer populations, leading to the emergence of Lyme disease.

Pathogen Adaptation

Microbes can evolve to become more dangerous or drug-resistant. Pandemic influenza often arises when different flu strains "reassort" in animals like pigs or ducks to create a brand-new virus that can infect humans.

Global Travel

Because humans can travel anywhere in the world in under 36 hours, localized outbreaks can quickly become global epidemics.

SARS (2003)

Spread to over 15 countries within just a few months via air travel.

West Nile Virus

Likely introduced to North America via an infected mosquito on an aircraft, after which it spread across the entire U.S. within five years.

Vector Movement

Changes in climate or trade can move disease-carrying insects to new areas. The Zika virus spread from Asia to the Americas as mosquito habitats expanded and international trade provided new transport routes

Early detection

the most critical step in stopping an outbreak before it becomes a pandemic

Molecular Tools

Techniques like real-time PCR (Polymerase Chain Reaction) allow scientists to identify a pathogen's genetic material within hours, even if it is a "new" variant.

GPHIN and HealthMap

These automated systems scan global news, social media, and online reports every hour to find early signals of disease outbreaks.

Metagenomic Sequencing

Large-scale sequencing of viral genomes can reveal how a disease like mpox is moving through connected regions in real-time.

Environmental Monitoring

In regions like the U.S.-Mexico border, scientists use advanced digital PCR to detect pathogens like the Valley Fever fungus in soil samples, warning communities of "hotspots" before infections peak.