Disease Detectives - General Terms

Natural History

The progression of a disease without treatment. It is characteristic of many diseases but may vary with different individuals. It is impacted by preventative and therapeutic measures.

Spectrum of Disease

The range of severity of a disease/illness.

Chain of Infection

Several connected steps that describe how a pathogen spreads.

Infectious Agent

An organism with the capacity to cause infection in a susceptible host.

Infectivity

The capacity to cause **infection** in a susceptible host.

Pathogenicity

The capacity to cause **disease** in a susceptible host.

Virulence

The severity of disease caused by a pathogen.

Reservoir

The origin of a pathogen.

Portal of Exit

How a pathogen exits the reservoir.

Gastrointestinal - Portal of Exit

Pathogens causing this type of disease often exits through feces or saliva.

Mode of Transmission

How a disease spreads-- either through contact, a vehicle, or vector.

Contact Transmission

This kind of transmission is comprised of direct/person-to-person, indirect/fomite, and/or droplet transmission.

Direct Contact

This method of transmission involves touching infected persons, kissing, and activities such as dancing.

Indirect Contact

This method of transmission is caused by coming in contact with a reservoir via inanimate objects (fomites). Combated by avoiding contact with fomites, not touching mucous membranes, using gloves/barriers when interacting w/ fomites, and disinfecting fomites.

Fomites

Anything an infected person touches which leaves a residue of contagious pathogen-- more difficult to avoid than direct contact transmission

Droplet Transmission

This kind of transmission involves being coughed, sneezed, and/or spit on. Only qualifies as this kind of transmission if the mucous droplets are traveling at the original velocity at which they left the mouth-- generally about 1 meter. Any further is considered airborne transmission.

Vehicle Transmission

Transmission by a medium such as food, air, and liquid-- the mediums serve as vehicles into the body.

Airborne Transmission

This kind of transmission is caused by liquid droplets that remain airborne as very small droplets.

Food-Borne Transmission

Any amount of pathogens found in food and not killed during processing can cause this kind of transmission.

Water-Borne Transmission

This kind of transmission often results from water contaminated by fecal matter-- often spreads gastrointestinal diseases.

Portal of Entry

An opening allowing the microorganism to enter the host; the route a pathogen takes to enter a host. Just as with the portals of exit, many pathogens have preferred portals of entry. Many pathogens are not able to cause disease if their usual portal of entry is artificially bypassed. Often the mucous membrane of the respiratory tract.

Susceptible Host

A person who cannot resist a pathogen invading the body, multiplying, and resulting in infection.

Epidemic

A disease widely occurring in a community within a short window of time. More widespread than an outbreak.

Outbreak

A sudden influx of a disease, more than normally expected. Involves a small area, such as a town or several neighboring towns.

Pandemic

A disease widely occurring over an entire country or the world.

Surveillance

The gathering of information-- collecting, analyzing, and interpreting large volumes of data from a variety of sources. “Is there a problem, if so, what is it?”

Passive Surveillance

This kind of surveillance involves healthcare providers, laboratories, hospitals, etc. reporting diseases to state/local officials.

Active Surveillance

This kind of surveillance involves health agencies actively contacting health providers, hospitals, and physicians for reports such as public health surveys, etc.

Sentinel Surveillance

This kind of surveillance involves regular reports of health events by health professionals selected to represent a geographical area/group.

Syndromic Surveillance

This kind of surveillance focuses on one or more symptoms rather than a physician-diagnosed or laboratory-confirmed disease. The data can be used to detect or anticipate outbreaks to mobilize a rapid response. Refers to methods relying on the detection of individual and population health indicators that are discernible before confirmed diagnoses are made. In particular, prior to the laboratory confirmation of an infectious disease, ill persons may exhibit behavioral patterns, symptoms, signs, or laboratory findings that can be tracked through a variety of data sources.

Risk

The probability that an individual will be affected by, or die from an illness within a stated time or age span. Interchangeably used with incidence.

Vector

A living organism that is able to transmit infectious pathogens between humans or from animals to humans.

Convalescent Vector

This kind of vector is able to spread a disease after recovery, although the vector may often think they are fully cured.

Incubatory Vector

This kind of vector transmits pathogens following infection but prior to showing symptoms.

Chronic Vector

This kind of vector is contagious for a long period of time.

Genetic Vector

This vector is an inherited yet asymptomatic disease trait.

Transient/Temporary Vector

This kind of vector is able to transmit an infectious disease for a short time frame.

Epidemiologist

This person counts cases or health events and categorizes them by time, place, and person. They divide the number of cases appropriately to calculate rates and compare disease rates over time for different groups. This is an umbrella term for a wide range of roles.

Epidemiologic Triad Model

Host → Agent → Environment

Case Definition

What an epidemiologist creates when they decide what defines a case. All further steps are based on this. It is a set of standards used to determine if a person has a certain health issue. Establishes clinical information about the disease/condition, characteristics of the affected people, the location/place, and the time during which the outbreak/condition occurred.

Casual Relationship

This is supported when there is existing evidence that a similar exposure causes a similar outcome.

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.

Endemic Disease

Present at a continuous level throughout a population/geographic area; constant presence of a health/agent condition (ex: the flu)

Etiology

The study of the cause of a disease.

Iatrogenic

An illness that is caused by a medication or physician.

Incubation Period

Time in between when a person comes into contact with a pathogen and when they first show symptoms or signs of disease.

Index Case

First patient in an epidemiological study, patient zero.

Morbidity

Rate of disease in a population.

Mortality

Rate of death in a population.

Plague

A serious, potentially life-threatening infectious disease that is usually transmitted through the bites of rodent fleas. Consists of three main forms-- bubonic, septicemic, and pneumonic.

Nosocomial Disease

An infection/disease that is acquired in a hospital.

Zoonosis

An infectious disease that is transmissible from animals to humans.

Symptomatic

Showing symptoms or signs of injury.

Asymptomatic

Showing no signs or symptoms, although can be a carrier of disease.

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.

Clinical Epidemiology

Studies patients in healthcare settings in order to improve the diagnosis and treatment of various diseases and the prognosis for patients already affected by a disease. Divided into infectious disease epidemiology and chronic disease epidemiology.

Natural History: Steps

Appropriate exposure/accumulation of factors → Incubation/latency period → Clinical disease (symptoms show) → Recovery, disability, or death

STI - Portal of Exit

Pathogens causing this type of disease often exits through the urethra or genitals.

Blood-borne Disease - Portal of Exit

Pathogens causing this type of disease often exit from anthropods (crustaceans, arachnids, centipedes, insects, etc.), needles, open wounds hypodermic syringes, butchered animals, and/or surgery on an infected persons.

Importance of a Case Definition

It is important to have this commonly established so that actual differences in cases can be observed rather than a variation in how the cases are being classified.

Public Health Surveillance

Ongoing, systematic collection, analysis, interpretation, and spreading of health data used to aid public health.

Field Investigation

Provides reports of clusters or cases to aid public health surveillance. Investigation can simply be a phone call to a health provider or a larger-scale effort to investigate the severity of the epidemic and its causes. Objectives can vary from the identification of unreported/unrecognized patients, identifying the source/vehicle of infection to control/eliminate it, to learning more about the natural history, clinical spectrum, descriptive epidemiology, and risk factors.

FATTOM

Describes the six favorable conditions required for the growth of foodborne pathogens.

F: Food (sufficient nutrients such as protein)

A: Acidity (pH of 4.6-7.5)

T: Time (Food should be removed from the danger zone within 2-4 hours by heating/cooling

T: Temp (41-135 F is “danger zone”)

O: Oxygen (Most pathogens need oxygen to grow-- aerobic)

M: Moisture (0.95-1.0 AW)

Timeline

Stage of susceptibility (exposure) → stage of subclinical disease (pathologic changes) → onset of symptoms → usual time of diagnosis → subclinical disease → recovery, disability, or death

Analytical Epidemiology

Uses comparison groups. Concerned w/ search for causes & effects, the why & how, used to quantify the association between exposures and outcomes & to test hypotheses. Can often provide enough information to take appropriate control and prevention measures.

Control/Comparison Group

Groups that are otherwise exactly the same as a the exposed group, but remain unexposed/unaffected by the disease. Used in outbreak investigations.

Experimental Epidemiological Study

In this type of study, the investigator uses a controlled process to *determine* the exposure for each individual (clinical trial) or community (community trial), & tracks the individual or community to detect the effects of the exposure. Often involves the use of placebos compared with other medications/methods of intervention (ie. vaccines, lifestyle intervention, etc.)

Observational Epidemiological Study

In this type of study, the investigator only *observes* the exposure and the disease status of each study participant. The two most common types are cohort & case-control studies, a third type is cross-sectional studies.

Retrospective Study

In this study, subjects are sampled and information on their past is collected (examines exposure to suspected risk/protection factors). Both the exposure and outcomes have already occurred in this study. Often used in investigations of disease in groups of easily identified people, such as at a school or wedding.

Prospective Study

Individuals are followed over time & data is collected as their characteristics/circumstances change (lower chance of bias)

Cohort Studies

Participants chosen based on exposure– epidemiologist records whether each study participant is exposed or not, and then tracks participants to see if they develop disease of interest (use relative risk), can be prospective or retrospective but always forward directionality (exposure known before outcome).

Case-Control Studies

Participants chosen based on disease status– investigators start by enrolling a group of people with disease (case patients) + a comparison group without disease (control) to compare previous exposure between groups (use odds ratio), retrospective and backward directionality (outcome known before exposure).

Cross-Sectional Study

A group of individuals from a population is enrolled, their exposures & health outcomes are measured at the same time. Generally measures the presence of the health outcome at that moment (“snapshot in time”/prevalence study) w/out regard to duration.

Random Error

The result of fluctuations around a true value due to the sample population. It is impossible to correct, but can be reduced by increasing the sample size and making more precise measurements (more trials, better devices, etc.)

Systematic Error

Any error besides random error, consistent and repeatable, often as a result of flawed equipment or experiment design. Data may remain usable, although not entirely accurate.

Human Error

Involves unintentionally harming the subject/patient.

Gross Error

Serious errors that cannot be attributed to systematic/random errors related to the sample, instrument, or procedure (ie. writing 10 as 100, etc.)

General Error

Either a systematic, random, or gross error.

Structured Error

This error is any error other than random error. It is usually consistent and repeatable and often occurs from flawed equipment or experiment design.

Instrumental Error

When inaccurate measurements are given by instruments.

Selection Bias

When the selection of participants for a study is affected by an unknown variable associated with the exposure and the outcome being measured.

Voluntary Bias

Bias in who chooses to participate in a study.

Attrition Bias

Systematic differences between people who leave and those who continue the study can sometimes introduce this kind of bias.

Response Bias

A wide range of tendencies for participants to give inaccurate/false responses, relevant in research involving self-reports

Information Bias

When bias is introduced through an error in measurement or observation.

Length Bias

Survival seems longer because more cases progress slowly.

Confounding Bias

Bias resulting from mixing effects of several factors.

Attention Bias

When subjects act differently because they know they’re being studied. Also known as the Hawthorne effect.

Overdiagnosis Bias

Survival seems longer than it is because of more nonfatal cases.

Neyman Bias

Missing of fatal cases, mild silent cases, and cases of improper follow-ups can introduce this kind of bias.

Sensitive case definition

Will detect many cases but may also count as cases individuals who do not have the disease.

Specific case definition

Is more likely to include only people who have the disease but is more likely to miss some cases.

Antigenic drift

When a virus undergoes a gradual change in genetic makeup, causing a different, but somewhat similar genetic makeup to the parent virus.

Antigenic shift

Occurs when a virus undergoes a sudden change in genetic makeup, creating a new strain.

Vaccine efficacy

Measured in a controlled clinical trial and is based on how many people who got vaccinated developed the ‘outcome of interest’ (usually disease) compared with how many people who got the placebo (dummy vaccine) developed the same outcome.

Vaccine effectiveness

A measure of how well vaccines work in the real world. Measured by observing how well the vaccines work to protect communities as a whole.

Sequelae

Aftereffect fo disease, condition/injury

Determinant

Anything that can change someone’s risk for disease

Disability

The restriction of one’s ability to perform an activity in the manner considered normal for a human being

Impairment

Any loss or abnormality of psychological, physiological, or anatomical structure or function