Disease Detectives

Epidemiology
Epidemiology is the study of distribution and determinants of health-related states in specified populations, and the application of this to control health problems.

Two Basic Types of Epidemiology

 * 1) Classical Epidemiology - population oriented, studies community origins of health problems related to nutrition, environment, human behavior, and the psychological, social, and spiritual state of a population. The event is more aimed towards this type of epidemiology.
 * 2) Clinical Epidemiology - studies patients in health care settings in order to improve the diagnosis and treatment of various diseases and the prognosis for patients already affected by a disease. These can be further divided into:
 * Infectious Disease Epidemiology - heavily dependent on laboratory support
 * Chronic Disease Epidemiology - dependent on complex sampling and statistical methods

Basic Epidemiology Terms
Cluster - an aggregation of cases over a particular period closely grouped in time and space, regardless of whether the number is more than the expected number

Outbreak - more cases of a particular disease than expected in a given area or among a specialized group of people over a particular period of time

Epidemic - large numbers of people over a wide geographical area affected

Pandemic - an epidemic occurring over several countries or continents and affected a large proportion of the population

Surveillance - The systematic and ongoing collection, analysis, interpretation, and dissemination of health data. The purpose of public health surveillance is to gain knowledge of the patterns of disease, injury, and other health problems in a community so that we can work towards their prevention and control.

Hill's Criteria for Causation
Nine criteria must be met to establish a cause-and-effect relationship. This is commonly known as Hill's Criteria for Causation:


 * 1) Strength of Association - relationship is clear and risk estimate is high
 * 2) Consistency - observation of association must be repeatable in different populations at different times
 * 3) Specificity - a single cause produces a specific effect
 * 4) Alternative Explanations - consideration of multiple hypotheses before making conclusions about whether an association is causal or not
 * 5) Temporality - cause/exposure must precede the effect/outcome
 * 6) Dose-Response Relationship - an increasing amount of exposure increases the risk
 * 7) Biological Plausibility - the association agrees with currently accepted understanding of biological and pathological processes
 * 8) Experimental Evidence - the condition can be altered, either prevented or accelerated, by an appropriate experimental process
 * 9) Coherence - the association should be compatible with existing theory and knowledge, including knowledge of past cases and epidemiological studies

Hill's Criteria for Causation Explanations and History

Epidemiological Triad
The traditional model of infectious disease causation. It is commonly known as the agent/host/environment triad. Includes three components:
 * 1) an external agent
 * 2) a susceptible host
 * 3) an environment that brings the host and agent together, so that disease occurs.

Basic Studies

 * 1) Ecological- comparisons of geographical locations
 * 2) Cross Sectional - a survey, "snapshot in time"
 * 3) Case-control - compare people with and without disease to find common exposures
 * 4) Cohort - compare people with and without exposures to see what happens to each
 * 5) Randomized controlled trial - human experiment

2*2 Table
Table which has two columns and rows for people with or without exposure and with or without disease; shows amount of people with each characteristic.

Using the 2*2 Table, we can calculate odds ratio and relative risk. These calculations allow comparisons between the control group and the group afflicted with the condition. One is the neutral value and means that there is no difference between the groups compared; when the value is greater than one it means that there has been some difference between the two groups, whether it was caused by bias, chance, or an actual relationship between the exposure and outcome is yet to be seen. The P-value tells us whether the results of the study can be used. The P-value is the measure of how confident you are that your findings are correct. You can only trust your findings to be correct if the P-value is less than .05.

Odds Ratio - used in case-control study, ad/bc Relative Risk - used in cohort study, (a/(a+b))/(c/(c+d))

Using Epi-Curves
An epi-curve is a histogram that shows the course of an outbreak by plotting the number of cases of a condition according to the time of onset.

Epi-Curves fall into three classifications:

Point source epidemics occur when persons are exposed to the same exposure over a limited, well define period of time. The shape of the curve commonly rises rapidly and contains a definite peak, followed by a gradual decline.



Continuous common source epidemics occur when the exposure to the source is prolonged over an extended period of time and may occur over more than one incubation period. The down slope of the curve may be very sharp if the common source is removed or gradual if the outbreak is allowed to exhaust itself.



Propagated (progressive source) epidemics occur when a case of disease serves later as a source of infection for subsequent cases and those subsequent cases, in turn, serve as sources for later cases. The shape of this curve usually contains a series of successively larger peaks, reflective of the increasing number of cases caused by person-to-person contact, until the pool of those susceptible is exhausted or control measures are implemented.



Chain of Infection
Agent leaves reservoir through portal of exit, and is conveyed by some mode of transmission, and enters the appropriate portal of entry to infect a susceptible host.




 * Agent - A microbial organism with the ability to cause disease.
 * Reservoir - A place where agents can thrive and reproduce.
 * Portal of Exit - A place of exit providing a way for an agent to leave the reservoir.
 * Mode of Transmission - Method of transfer by which the organism moves or is carried from one place to another.
 * Portal of Entry - An opening allowing the microorganism to enter the host.
 * Susceptible Host - A person who cannot resist a microorganism invading the body, multiplying, and resulting in infection.

Chain of Infection: Diagram and Explanation

Characteristics of Agents

 * 1) Infectivity - capacity to cause infection in a susceptible host
 * 2) Pathogenicity - capacity to cause disease in a host
 * 3) Virulence - severity of disease that the agent causes to host

Modes of Disease Transmission

 * Direct transmission - immediate transfer of agent from a reservoir to a susceptible host by direct contact or droplet spread.
 * Direct contact - occurs through kissing, skin-to-skin contact, and sexual activity.
 * Droplet spread - direct transmission by direct spray over a few feet, before droplets fall to ground.
 * Indirect transmission - agent is carried from reservoir to a susceptible host by suspended air particles, vectors, or vehicles.
 * Vectors - animate intermediaries (such as fleas, flies, and mosquitoes) which carry the agent through mechanical means.
 * Vehicles/Fomites - inanimate intermediaries (objects) that carry agent


 * Mechanical transmission - no multiplication or change of the agent within the vector
 * Biological transmission - when the agent undergoes changes within the vector, and the vector serves as both an intermediate host and a mode of transmission

Disease Prevention

 * Primary prevention - early intervention to avoid initial exposure to agent of disease preventing the process from starting
 * Secondary prevention - during the latent stage (when the disease has just begun), process of screening and instituting treatment may prevent progression to symptomatic disease
 * Tertiary prevention - during the symptomatic stage (when the patient shows symptoms), intervention may arrest, slow, or reverse the progression of disease

Ten Steps to Investigating an Outbreak

 * 1) Prepare for Field Work
 * 2) Establish the Existence of an Outbreak
 * 3) Verify the Diagnosis
 * 4) Define and Identify Cases
 * 5) Describe and Orient the Data in Terms of Person, Place, and Time
 * 6) Develop Hypotheses
 * 7) Evaluate Hypotheses
 * 8) Refine Hypotheses and Carry Out Additional Studies
 * 9) Implement Control and Prevention Measures
 * 10) Communicate Findings

Ten Steps to Outbreak Investigation - Explanation of Steps