Community Medicine
Part 1 Terminology relevant to infectious diseases
Prof. Salwa Tayel [email protected] Community Medicine Department, College of Medicine, Alexandria University Infection
• It is the entry, development and/or multiplication of an infectious agent in the body of man or animal.
• Infection is not synonymous with infectious disease
• The result of infection may be:
– inapparent (without relevant symptoms or signs) or
– apparent (with relevant symptoms and signs).
Prof. Salwa Tayel 3 Infectious Disease
• Infectious disease:
A clinically manifest disease of man or animal resulting from infection.
Prof. Salwa Tayel 4 Concept of Infection
Death Clinical Disease Severe Disease
Mild Disease
Subclinical Infection without Clinical Illness
Exposure without Infection
Prof. Salwa Tayel 5 Contamination
• Contamination is the presence of living infectious agents on the
exterior surface of the body or on the clothes or articles of the person
or on any inanimate object in the environment including water and
food.
Prof. Salwa Tayel 6 Communicable disease
• It is an illness caused by an infectious agent or its toxic product which
can be transmitted directly or indirectly or through a vector from the
reservoir to a susceptible host.
Prof. Salwa Tayel 7 Contagious disease
• Contagious disease: It is a communicable disease that is easily transmitted from person to person by
– direct contact examples: measles, influenza, HIV
– indirect contact examples: typhoid, cholera, hepatitis
Prof. Salwa Tayel 8 Non contagious disease
• Non contagious disease: It is a communicable disease that is not transmitted through contact with infected persons but it is transmitted through:
– vectors (e.g. malaria)
– spores (e.g. tetanus)
– toxins produced by an organism (e.g. botulism and staphylococcal food poisoning).
Prof. Salwa Tayel 9 Incubation period
• Incubation period: It is the time interval from initial contact with an infectious agent and the appearance of first signs or symptoms associated with the infection.
• Knowledge of the incubation period is important for surveillance and quarantine measures for contacts of cases of communicable diseases. Also to apply preventive measures to abort or modify the attack.
Prof. Salwa Tayel 10 Natural History of Disease
Exposure Time
Dynamics of Incubation Symptomatic disease period period --recovered Susceptible
Onset of Time symptoms
Prof. Salwa Tayel 11 Quarantine
• Quarantine: Restriction of the activities of well persons or animals who
have been exposed to a case of communicable disease during its period
of communicability (i.e., contacts) to prevent disease transmission
during the incubation period if infection should occur.
Prof. Salwa Tayel 12 Infectiousness
• Latent period: It is the period between exposure and the onset of the infectious period.
• Infectious period or period of communicability: it is the time during which the infectious agent could be transmitted directly or indirectly from an infected person to another, from infected animals to humans or from an infected person to animals, including arthropods. • Knowledge of infectious period is important to determine the duration of isolation of cases with communicable diseases.
Prof. Salwa Tayel 13 Infectious period or period of communicability (PC) Exposure Dynamics of Noninfectious Latent Infectious infectiousness period Period -- immune (PC) Susceptible E Time
Dynamics of Incubation Symptomatic disease period period --recovered Susceptible Onset of symptoms Time
Timelines for Infection and Disease Prof. Salwa Tayel 14 Infectious period or period of communicability (PC) Exposure Dynamics of Noninfectious Latent Infectious infectiousness period Period -- immune (PC) Susceptible E Time
Dynamics of Incubation Symptomatic disease period period --recovered Susceptible Onset of symptoms Time
Timelines for Infection and Disease Prof. Salwa Tayel 15 Infectious period or period of communicability (PC) Exposure Dynamics of Latent Infectious Chronic Carrier infectiousness period Period For life (PC) Susceptible E Time
Dynamics of Incubation Symptomatic disease period period carrier Susceptible Onset of symptoms Time
Timelines for Infection and Disease Prof. Salwa Tayel 16 Isolation
• Isolation: It is applied to a sick person or animal and it refers to the
separation for the period of communicability in such a place and under
such conditions to prevent or limit direct or indirect transmission of
infectious agent from those infected to those who are susceptible to
the infection or who may spread the agent to others.
Prof. Salwa Tayel 17 Levels of Disease in the population
Increasing amount of disease
Pandemic
Epidemic
Endemic
Sporadic
Prof. Salwa Tayel 18 Sporadic disease
• It is the occurrence of few cases of the disease who are separated
widely in terms of geographic location and time of occurrence and
showing no connection to each other.
Prof. Salwa Tayel 19 Endemic disease
• It is the constant presence of a disease or the constant presence of an infectious agent within a given geographic area or population group.
• It may also refers to the usual prevalence of a given disease within such area or group.
• Examples: Bilharziasis in Egypt, malaria in the tropics and typhoid, hepatitis in many developing countries
Prof. Salwa Tayel 20 Epidemic
• Epidemic:
• The occurrence in a community or region of cases of an illness, clearly in
excess of its normal expectancy for the same time of the year.
Prof. Salwa Tayel 21 Endemic vs. Epidemic
Hyperendemic of Cases of a Disease a of Cases of
umber Epidemic
N Endemic
Time
Prof. Salwa Tayel 22 Outbreak
• Outbreak:
• It is a localized epidemic affecting large numbers of individuals sharing similar experience
• Example: outbreak of food poisoning in an institution
Prof. Salwa Tayel 23 Pandemic
Pandemic: An epidemic occurring over a very wide geographical area, crossing international boundaries, and usually affecting a large number of people
Examples:
Influenza A H1N1 in 2009
COVID-19 starting December 2019
Prof. Salwa Tayel 24 Reservoir
• Reservoir: It is any person, animal, arthropod, plant, soil, or substance, or combination of these in which an infectious agent normally lives and multiplies, on which the agent depends primarily for survival, and where the agent reproduces itself in such a manner that it can be transmitted to a susceptible host.
Prof. Salwa Tayel 25 Carrier
• Carrier: A human carrier is a person who harbors the infectious agent, without showing any signs or symptoms and capable to transmit the infectious agent to healthy community members.
Prof. Salwa Tayel 26 Zoonosis
• Zoonosis or zoonotic disease:
A primarily disease of animals transmitted under natural conditions from vertebrate animals to humans (e.g. rabies and plague).
Prof. Salwa Tayel 27 Herd immunity
• Herd immunity: It is the immunity of a group of people or a community (Herd).
• The resistance of a group of people or a community to a particular disease depends on the immunity acquired by a high proportion of individual members of this group or community that provides protection to non-immune individuals.
Prof. Salwa Tayel 28 Herd Immunity Threshold
• Herd immunity threshold (HIT): It is the level of immunity of a population, indicated by the proportion of immune persons, above which the incidence of the disease decreases.
• The threshold of herd immunity is variable from one disease to another
– It is nearly 80% for smallpox while
– It is estimated to be 60% of COVID-19
Prof. Salwa Tayel 29 Utility of the Concept of Herd immunity
• The target of vaccination programs is to increase the vaccination coverage above the level of (HIT)
• This will decrease the disease transmission and provide protection to the un-vaccinated individuals.
• As herd immunity becomes diluted by new individuals born susceptible, vaccination programs are targeting newborns and children to maintain herd immunity level above the critical level or (HIT).
Prof. Salwa Tayel 30 Herd Immunity Threshold
Prof. Salwa Tayel 31 Prof. Salwa Tayel 32