Epidemiology v1.2 unit 2

Unit 2: Terminologies used in
Epidemiology
(4 Hours)
Draft Version 1.1
Upendra Raj Dhakal
Lecturer: Valley College of Technical Sciences
9849110689
urdhakal@gmail.com

Infection
• The invasion and multiplication of microorganisms such as bacteria,
viruses, fungi that are normally not present within the body.
• Parasites and Arthropods causes infestation. E.g.. Mosquito – malaria, lice
causes itching
• Infection might have no symptoms/subclinical(latent), or clinical symptoms
might be present.
• Infection might be localized or may spread through the blood or lymphatic
vessel to become systemic (body wise)
• Microorganisms (generally called as pathogens), that live naturally within
the body does not cause infection, but might occur if those pathogens
change their location
• When we have weak immune system, there is high chance of infection.
Draft Version_1.2 (Feedbaack Welcomed)

Chain of Infection

Primary and Secondary Infection
• A primary infection is the first time you are exposed to
and infected by a pathogen.
• During a primary infection, your body has no innate defenses against
the organism, such as antibodies.
• A secondary infection can occur when a different infection, known as
the primary infection, makes a person more susceptible to disease.
• It is called a secondary infection because it occurs either after or
because of another infection. In other words, it is secondary to
that infection.

Incubation and window period
• Incubation period is the time elapsed between exposure to
a pathogenic organism, a chemical, or radiation, and when symptoms and
signs are first apparent.
• In a typical infectious disease, incubation period signifies the period taken
by the multiplying organism to reach a threshold necessary to produce
symptoms in the host.
• E.g.. ……………….?.............
• Window period is the time elapsed between first exposure to a pathogenic
organism, a chemical, or radiation, and its detection in laboratory.
• Incubation period is generally larger than window period, but it is not
necessary that incubation period is always larger than window period, esp.
when diseases are diagnosed by chance while diagnosing other disease.
• E.g.. …………?..............

Signs and Symptoms of Infection
• Fever (This is sometimes the only sign of an infection) causing headache
and decrease in appetite.
• Chills and sweat
• Feeling a malaise
• Viral infections are not treated with antibiotics, rather use of antibiotics
increases the risk of antibiotic resistance.
• Fluid drainage
• Continual or increased pain
• Redness or swelling
• Etc.…

Disease
• A condition of the body or some parts or organ of the body in which
its functions are deranged.
• It is a mal – adjustment of human organism to the environment.
• It is deviation from normal body function.

Communicable disease
•An illness due to a specific infectious agent or its toxic
products that arises through transmission of that
agent or its products from an infected person, animal
or inanimate reservoir to a susceptible host; either
directly or indirectly through an intermediate plant or
animal host, vector or the inanimate environment.

Contd …
• A communicable disease is one that is spread from one person to
another through a variety of way that include: contact with blood and
bodily fluids, breathing in an airborne virus, bite of an insect
• Communicable disease is caused by bacteria, viruses, parasites or
fungi.
• Communicable disease can be transmitted directly (human – human),
or indirectly.
• Organisms that cause communicable disease are known as pathogens

Case
• Cases are the group of people who have disease.
• In epidemiology, cases are studied either they contained risk factors
or not- in case – control studies (In cohort and RCT, we don’t use the
case)
• During study, cases are made homogenous as far as possible.
• Case definition: It is a set of criteria for determining whether a person
should be classified as being affected by the disease under
investigation. It includes sensitivity, not specificity.

Contd …
• Index case
• A case that comes to the attention of public health authorities
• Primary case
• Person who acquires the disease from an exposure for the first time.
• Primary case comes in contact with the population and spreads the disease
• Secondary case
• Person who acquires the disease from an exposure to the primary case

Host
• Host are animals who can get the disease.
• We are human host, or simply host.
• Host factor play major role in determining the outcome of an
individuals exposure to infection.
• Host factor can be classified as:
• Demographic: age, gender, …
• Biological: genetic, biochemical level, blood group and enzyme, cellular
constituents, …
• Social and economic: socio-economic, education, occupation, stress, …
• Lifestyle: alcohol, drugs, smoking, …

Agent
• Substance, living or non living, or a force, tangible or intangible, excessive
presence or relative lack of which may initiate or perpetuate a disease
process.
• A disease might have single agent, number of independent alternative
agents, complex or two or more factors.
• Types of agents:
• Biological agents: bacteria, fungi, virus, protozoa, etc..
• Nutrient agents: Carb, protein, Fat, vitamins, minerals and water
• Physical agents: heat, cold, humidity, radiation, noise, electricity, etc..
• Chemical agent: Endogenous (urea, serum bilirubin, ketones, etc..) and Exogenous
(allergens, metals, fumes, dust, gases, …)
• Mechanical agents: Crushing, tearing, sprain, dislocation and even death
• Absence/Insufficiency or excess: hormones, chromosomal factors, immunological
factors, etc..

Environment
• Extrinsic factors that affect the agent and the opportunity for
exposure.
• Environmental factors includes:
• Physical factors: geology and climate, …
• Biological factors: insects, …
• Socio – economic factors: crowding, sanitation, availability of health services

Relation between agent host and
environment (Epidemiological triad)

Carriers
• Carrier is an organism with inapparent infection, who is capable of
transmitting the pathogens to others.
• Asymptomatic or passive or healthy carrier are those who never
experience symptoms despite being infected.
• Convalescent carriers are those who have recovered from their illness
but remain capable of transmitting disease to others.
• Chronic carriers are those who continue to harbor a pathogen, such
as HBV
• Humans, and any animals or even plants can act as carrier.

Vectors
• An organism, typically a biting insect or tick, that transmits a disease
or parasite from one animal or plant to another.
• Organism that carries and transmits an infectious pathogen into
another living organism;
• Sometimes vectors could be an inanimate medium of infection such
as dust particles.
• Vectors carry pathogens/agents and transmits to humans that may
result a disease condition.
• E.g.. Mosquito, housefly, louse, tick, fleas, etc..

Reservoir
• Any person, animal, plant, soil or substance in which an infectious
agent normally lives and multiplies.
• The reservoir typically harbors the infectious agent without injury to
itself and serves as a source from which other individuals can be
infected.
• The infectious agent primarily depends on the reservoir for its
survival.
• It is from the reservoir that the infectious substance is transmitted to
a human or another susceptible host.

Contd …
• In infectious disease ecology and epidemiology, a natural reservoir,
also known as a disease reservoir or a reservoir of infection, is the
population of organisms or the specific environment in which an
infectious pathogen naturally lives and reproduces, or upon which the
pathogen primarily depends for its survival.
• E.g.. Humans, any animals or even plants.

Incubation Period
Refer previous slide (5)

Endemic
• It is a condition in
a population when that infection is
constantly maintained at a baseline
level in a geographic area without
external inputs.
• Each person who becomes infected
with the disease must pass it on to
one other person on average.

Outbreak and Epidemic
• An outbreak is the sudden occurrence of a disease in a community, which
has never experienced the disease before or when cases of that disease
occur in numbers (just) greater than expected in a defined area.
• When disease occurs in smaller scale, it is known as outbreak. When the
scale significantly increases, it is termed as epidemic. Many times,
outbreak and epidemic are used interchangeably.
• Before epidemic, disease must be out break.
• An increase in incidence above the expected in a defined geographic area
within a defined time period.
• It is the occurrence of an infectious disease clearly in excess of normal
expectancy, and generated from a common or propagated source.

Contd …
• To declare epidemic, there should be threshold meet of baseline rate
of incidence.
• Rare disease have lower threshold, whereas common disease have
higher thresholds
• It is not necessary that communicable diseases are only epidemic.
• E.g.. Meningococcal infection if have attack rate of 15 cases per
100000 people for two consecutive week

Types of epidemics
• Common Source outbreak: Affected individuals have an exposure to
common agent.
• If exposure is singular, and cases develop over a single exposure and
incubation course, it is point source outbreak.
• If the exposure is continuous, it is known as continuous outbreak.
• If the exposure is variable, it is known as intermittent outbreak.
• Propagated outbreak: In a propagated outbreak, the disease spreads
from person to person making affected individuals as an independent
reservoir for disease transmission. i.e. multiple organisms carry
disease agents to transmit (propagation)

Sporadic
• Occurring at irregular/haphazard intervals or only in a few places;
scattered or isolated.
• Diseases that are seen occasionally, and usually without geographic
concentration, are called sporadic disease.
• The cases of sporadic are few and separated widely in time and place
that they show no or little connection with each other, nor a
recognizable common source of infection.
• Sporadic disease might lead to epidemic.
• E.g.. Tetanus, rabies, and plague

Pandemic
• Pan = all, demos = people
• Pandemic: A worldwide epidemic affecting an exceptionally high
proportion of the global population.
• Pandemic spreads across a large region; for instance multiple continents, or
even worldwide.
• Pandemic = widespread + epidemic
• Increased likelihood of pandemic because of global travel and integration,
urbanization, changes in land use, and greater exploitation of natural
environment.
• E.g.. Black death/Great plague: Plague killed 75 million people in 1350;
Smallpox in Australia just before the colonization of Britain killed 50% of
the indigenous Australians; HIV; smallpox, Ebola, SARS; Cholera
Further: https://www.ncbi.nlm.nih.gov/books/NBK525302/

Lets understand this graph

Enzootic
• Endemic in animals
• An enzootic is a condition, where there is a constant presence of
disease in animal population
• In epizoology, an infection is said to be enzootic in a population when
the infection is maintained in the population without the need of
external inputs
• It is similar to endemic in in humans

Exotic disease
• Unusual or exiting condition because of coming (or appeared to
come) from far, specially from tropical and sub tropical areas.
• It is less prevalent to temperate climate due to the coldness, which
forces the control of infection due to the hibernation of reservoirs.
• It is also known as tropical disease
• E.g.. Ebola/hanta, Naegleria fowleri (brain eating amoeba), kuru
disease/laughing sickness (due to cannibalism, esp. brain), Measles,
Malaria, Leprosy, STI, Worms, Scabies, etc.

Zoonosis
• Zoonosis is any disease or infection that is naturally transmissible from
vertebrates animals to humans.
• Animals play an essential role in maintaining zoonotic infections in nature.
• Zoonoses may be bacterial, viral, or parasitic, or may involve
unconventional agents.
• As well as being a public health problem, many of the major zoonotic
diseases prevent the efficient production of food of animal origin and
create obstacles to international trade in animal products.
• E.g.. Swine Influenza (H1N1), Avian Influenza (H5N1), Rabies, Anthrax,
Brucellosis, Ebola,

Epornithic
• Epidemic in birds
• An epornithic is a condition, where there is an outbreak of disease in
birds population
•
• It is similar to epidemic in humans

Contamination
• Contaminants are the substances (desired or undesired) in any
natural environment.
• Contaminants are specific portion of matter or body that is not
wanted.
• When contaminants are harmful, than they are known as pollutants.
Pollutants might be pathogens or non pathogens/chemicals.
• Contamination is the presence of a constituent, impurity, or some
other undesirable element in a natural environment.
• E.g.. Water can be contaminated with iodine, but if there is presence
of urea, it is polluted.

Infestation
• Infestation is the state of being invaded or overrun by pests, parasites
or arthropods
• Parasites and Arthropods causes infestation. E.g.. Mosquito – malaria,
lice causes itching
• Infestation can be:
• External: External (or ectoparasitic) infestation is a condition in which
organisms live primarily on the surface of the host and includes those
involving mites, ticks, lice and bed bugs.
• Internal: An internal (or endoparasites) infestation is a condition in which
organisms live within the host and includes those involving worms. E.g..
Roundworms.

Contagious disease
• Contagious is derived from the word contagion, meaning contact
• Contagious disease are those which gets transmitted through direct
mode of transmission, like through physical contact and body
secretions.
• Non contagious infections need some medium for transmission, and
thus are transmitted indirectly.
• Presently, contagious disease are also identified as any communicable
or infectious disease.
• Often it is used to know very infectious, easily transmitted, severe
communicable disease.

Nosocomial infection
• Nosocomial = Hospital originated
• Nosocomial infections are those Infection acquired in hospital after or from
48 hours after admission.
• A hospital-acquired infection may be defined as "any clinically recognizable
microbiological disease that affects the patient as a consequence of his
being admitted to hospital or attending for treatment, or the hospital staff
as a consequence of their work, whether or not the symptoms of the
disease appear while the affected person is in the hospital".
• Causes and example: UTI, frequent antimicrobial resistance, central venous
catheter, Nosocomial pneumonia, Surgical site infection.
• Iatrogenic Infection: Due to the activity of physician or therapy.

Opportunistic Infection
• An opportunistic infection is an infection caused
by pathogens (bacteria, viruses, fungi, or protozoa) that take
advantage of an opportunity not normally available.
• It is seen in immune compromised patients.
• Immunodeficiency might occur due to: Malnutrition, Fatigue,
Recurrent infections, chemotherapy, genetics, etc.
• E.g.. Any infectious agent can act as opportunistic infectious agents.

Iatrogenic Infection
• Iatrogenic = Related to illness caused by medical examination or
treatment.
• Iatrogenic disease is the result of diagnostic and therapeutic
procedures undertaken on a patient.
• Iatrogenic can be infectious as well as non infectious
• With the multitude of drugs prescribed to a single patient adverse
drug reactions are bound to occur – which are non infectious.
• When Iatrogenic is contagious, it is known as iatrogenic infection
• Any pathogens can act as iatrogenic agents to cause disease.

Surveillance/Survey
• Close observation, especially of a suspected disease.
• It is the monitoring of behavior, activities, or other changing information
for the purpose of influencing, managing, directing, or protecting people.
• Public health surveillance is the continuous, systematic collection, analysis
and interpretation of health-related data needed for the planning,
implementation, and evaluation of public health practice.
• Surveillance must:
• serve as an early warning system for impending public health emergencies;
• document the impact of an intervention, or track progress towards specified goals;
and
• monitor and clarify the epidemiology of health problems, to allow priorities to be set
and to inform public health policy and strategies

Prevention
• Prevention typically consists of methods or activities that seek to reduce or
deter specific or predictable problems, protect the current state of well-
being, or promote desired outcomes or behaviors.
• Five Levels of Prevention:
• Primordial Prevention: Reduce risk factors themselves by healthier lifestyle
• Primary prevention: Preventing before disease occurs. E.g. immunization
• Secondary Prevention: Early Diagnosis, Treatment and disability limitation
• Tertiary prevention: Restoration and rehabilitation
• Quaternary prevention: Mitigating or avoiding unnecessary or excessive
interventions
• Understanding the natural history of disease, appropriate prevention
intervention can be done.

Control
• The power to influence or direct people's behavior or the course of
events
• Reduction in the incidence, prevalence, morbidity or mortality of an
infectious disease to a locally acceptable level
• It includes:
• Control
• Elimination
• Eradication, and
• Extinction

Certification of eradication
• WHO declares the eradication.
• Small pox is only one human disease eradicated ever
• Polio is underway for the declaration of eradication
• Rinderpest is zoonotic disease that has been declared eradicated.
• There are some criteria to declare the disease as eradicated:
(Assignment)
• ###
• ###
• ###

Criteria for Eradication - Assignment

Include following points for assignment
• Extreme care and detail procedure need to be followed.
• Biological factors determining eradication: ….???...
• Technical factors determining eradication: ….???...
• Specific disease should be identified clearly and accurately
• Eradication Initiative should be done.
• Detail lifecyle and intervention plan should be developed
• For Human disease: All host (other than humans) and reservoirs must be free from pathogens.
• For Animal disease: Easily identifiable reservoir.
• Availabe, acecssable (and affordable) interruption of disease transmission
• Routine vaccination for ###time###
• Global comitteement and action.
• Disease must not appear for ….?.... years continuously after eradication.

Screening
• a strategy used in a population to identify the possible presence of an as-
yet-undiagnosed disease in individuals without signs or symptoms.
• This can include individuals with pre-symptomatic or unrecognized
symptomatic disease.
• A screening test is done to detect potential health disorders or diseases in
people who do not have any symptoms of disease.
• The goal is early detection and lifestyle changes or surveillance, to reduce
the risk of disease, or to detect it early enough to treat it most effectively.
• Universal screening: Screens all; Case finding: Screens small group.
• Screening is not done to diagnose disease
• Screening have high chance of giving false positive or false negative result.

Thank You

Epidemiology v1.2 unit 2

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Epidemiology v1.2 unit 2