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Epidemiology
of infectious diseases
Kolářová M., EPI Autumn 2018
EPIDEMIOLOGY
is the study (scientific, systematic, data-driven)
of the distribution (frequency, pattern)
and determinants (causes, risk factors)
of health-related states or events (not just diseases)
in specified populations (patient is community,
individuals viewed collectively),
and the application (since epidemiology is a discipline
within public health) of this study to the control of health
problems.
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In tracking a disease outbreak, epidemiologists may
use any or all of three types of investigation:
a) descriptive epidemiology
is the collection of all data describing the occurrence of the disease
b) analytical epidemiology
attempts to determine the cause of an outbreak
c) experimental epidemiology
tests a hypothesis about a disease or disease treatment in a group
of people.
 Incidence - the incidence rate refers to the number of new
cases of a disease in a given population over a period of time.
 Prevalence - the prevalence rate refers to the number of total
cases of a disease in a given population at a specific time.
 Sporadic disease Disease that occurs
occasionally in a population.
 Endemic disease Disease constantly present in a
population.
 Epidemic disease Disease acquired by many
hosts in a given area in a short
time.
 Pandemic disease Worldwide epidemic.
Occurrence of Disease
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Epidemiologic investigations are
largely mathematical
descriptions of persons in groups,
rather than individuals.
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Epidemiologists arrange their data in various ways,
depending on what aspect of the information they want
to emphasize.
One of the most powerful tools an
epidemiologist can use is case reporting:
reporting specific diseases to
* local,
* state and
* national health authorities,
who accumulate the data
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Modern infectious disease
epidemiology
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Colonisation and contamination of humans
by micro-organisms.
Many parts of the body are colonized by normal flora, which
can be the source of endogenous infection. Large
numbers of micro-organisms are found in moist areas of
the skin (e.g. the groin, between the toes), the upper
respiratory tract, the digestive tract (e.g. the mouth, the
nasopharynx), the ileum and large intestine, the anterior
parts of the urethra and the vagina.
Other routes are interhuman transmission of infections and
exposure to exogenous contamination.
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Infectious diseases result from the interaction of
agent, host, and environment.
More specifically - spreding of infections occurs when:
the agent leaves its 1.reservoirs or host (source) trough a portal of exit,
is conveyed 2.by some mode of transmissin
and enters trough an appropriate portal of entry to infect
3.a susceptible host.
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THE CAUSATIVE AGENT OF INFECTION
(viruses, Chlamydia, rickettsia, mycoplasma, spirochete, bacteria, prions, fungi, protozoa helminthes)
1. the presence of rezervoir (source) of infection
2. the way of transmission
3. the susceptibility of the population or its individual member
to the organism concerned
THE INFECTION
= 1. source of infection
. . . .
. .
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1. the presence of source of
infection
is the site or sites in which a disease agent normally lives and
reproduces.
May be classified as:
- human - at the ende incubation period,
if is ill,
reconvalescent,
carriers – healthy,
chronic diseases
- animals - at the ende incubation period,
if is ill,
carriers – healthy,
reconvalescent, chronic
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2. the metod of transmission
A/ direct contact
touching, kissing or sexual intercourse (Staphylococcus spp.,
Gonococcus spp.,HIV …),
- vertical transmission – from mother to fetus (VHB, VHC, HIV,
listeria, rubella, cytomegalovirus…)
B/ indirect contact
- inhalation of droplets containing the infectious agents (TBC,
measles, influenza…)
- ingestion of food or water that is contaminated (salmonella,
giardia, Norwalk virus, VHA….)
- biological transmission by insects (malaria, borellia….)
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Organisms vary in their capacity to survive in the
free state and to withstand adverse
environmental conditions, for example:
* heat, cold, dryness.
Sporo-forming organisms, such as tetanus bacilli
which can survive for years in a dormant state,
have a major advantage over an organisms like
the gonococcus which survive for only a very
short time outside the human host.
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Routes of transmission
 Air
 Food, Drink or Water
 Direct or indirect contact
* Transplacental
 Insects (Artropods)
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Main portals of entry
 Respiratory tract
 Gastro-intestinal tract
 Genito-urinary tract
 Direct break through skin
* surgical and wounds
 Direct into blood via needles/catheters
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3. the susceptibility of the population or its individual
member to the organism concerned, and the characteristic
of the organism itself.
Host factors :
Non specific immunity
Barrier action (natural barrier)
External barrier:
skin , mucosa
Secretion of skin and mucosa
Accessory organ
Internal barrier: placenta, blood-brain barrier
Phagocytosis
Humoral action :
Complement, Lysozyme, Fibronection, Cytokines.
Specific immunity
Humoral immunity
Immunoglobulin: IgG, IgM, IgE, IgA, IgD
Cell mediated immunity
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THE CAUSATIVE AGENT OF INFECTION (bacteria, viruses, fungi, prions, protozoa)
1. the presence of rezervoir (source) of infection
man, animal at the ende of incubation period
acute stage
cariers
2. the way of transmission A/ direct contact
touching, kissing or sexual intercourse (Staphylococcus spp., Gonococcus spp.,HIV …),
- vertical transmission – from mother to fetus (VHB, VHC, HIV, listeria, rubella, cytomegalovirus…)
B/ indirect contact
- inhalation of droplets containing the infectious agents (TBC, measles, influenza…)
- ingestion of food or water that is contaminated (salmonella, giardia, Norwalk virus, VHA….)
- biological transmission by insects (malaria, borellia….)
3. the susceptibility of the population or its individual members to the organism
concerned Host factors : a g e , n u t r i t i o n, g e n e t i c s
i m m u n i t y – natural (nonspecific, specific),
- acquired (after vaccination)
THE INFECTION
= 1. source of infection
……….
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THE CAUSATIVE AGENT OF INFECTION
(viruses, Chlamydia, rickettsia, mycoplasma, spirochete, bacteria, prions, fungi, protozoa helminthes)
1. the presence of rezervoir (source) of infection
2. the way of transmission
3. the susceptibility of the population or its individual member
to the organism concerned
THE INFECTION
= 1. source of infection
. . . .
. .
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ANTI - EPIDEMIC MEASURES
1. the presence of rezervoir (source) of infection
ISOLATION
2. the way of transmission HAND WASHING, LINEN WASHING, CLEANING,
GOOD PREPARING OF FOOD, SAFE WATER……..,
DISINFECTION, STERILIZATION
3. the susceptibility of the population or its individual member
to the organism concerned VACCINATION
THE INFECTION
= 1. source of infection
. . . .
X
x
XREDUCING THE INCIDENCE,
ELIMINATION,
ERADICATION
Remembering an Old
Disease
Smallpox
Face lesions on boy with smallpox.
Public Health Images Library (PHIL) ID # 3.
Source: CDC/Cheryl Tyron
Smallpox recognition
card, c.1973, courtesy
Dr. Damodar Bhonsule,
Panjim, Goa, India.
Smallpox lesions on skin of trunk.
Picture taken in Bangladesh, 1973.
Public Health Images Library (PHIL) ID # 284. Source: CDC/James Hicks
Rural vaccinator in United Provinces, British India, c.1930, private collection of Dr. Sanjoy Bhattacharya
Variola virus, which causes smallpox, was once the scourge of the world.
This virus passes from person to person through the air.
A smallpox infection results in fever, severe aches and pains, scarring
sores that cover the body, blindness in many cases, and, often,
death. There is no effective treatment.
Although vaccination and outbreak control eliminated smallpox in the
United States by 1949, the disease still struck an estimated 50 million
people worldwide each year during the 1950s.
In 1967, the World Health Organization (WHO) launched a massive
vaccination campaign to rid the world of smallpox —and succeeded.
The last natural case of smallpox occurred in Somalia in 1977.
Ali Maow Maalin, cook twenty-three of the hospitals in the Somali Merce.
He contracted when he showed the path of the ambulance chauffeur who drove
two sick children to camp insulation.
In 1978 was ill photographer Medical School in Birmingham, England. She was killed
by a virus that escaped from a neighboring lab.
Mr. John Wickett, of the World Health Organization,
with the last person to have contracted – and survived –
naturally occurring smallpox in Somalia.
(1977), courtesy Mr. John Wickett.
Eradication of smallpox
Czech experts
A key figure in the global eradication program
smallpox was prof. MUDr. Karel Raska, MD.,
who drove in the sixties division
Communicative Diseases of the WHO
Secretariat in Geneva.
He promoted the establishment of a new, independent
units "Eradication of smallpox"and ensure its initial financial
and material support, not only in Geneva,
but also in regional offices of WHO.
With its support of the program also attended the 20
Czechoslovak health professionals (14 Czechs and Slovaks
6), mainly epidemiologists.
They participated in both the preparation methodology and
procedures, thus working directly in infested areas.
Smallpox eradication was
officially announced
at the 33rd General Assembly WHO
8. May 1980.
Tuesday, March 8, 2011; 9:41 AM
Milestones in the eradication of smallpox
1789 Edward Jenner invents a smallpox vaccine.
1966 The World Health Organization (WHO) launches a massive global
campaign to eradicate smallpox.
1972 Smallpox vaccinations are discontinued in the United States.
1975 and 1977 The last cases of the two known variants of smallpox occur in
the world, in Bangladesh and Somalia.
1978 Two people are sickened in a lab accident in England; one dies.
1980 The WHO declares smallpox eradicated.
1991 Smallpox virus DNA is mapped.
1999 The WHO sets this deadline, by which remaining lab stocks of the virus are to
be destroyed. The deadline will be postponed again and again.
2003 Millions of doses of vaccine are produced to hedge against a biological
attack.
2011 WHO's decision-making body will meet in May to again vote on whether to kill
the remaining live viruses.
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If the epidemiology is know, we can
interfere with transmission:
„BREAKING THE CHAIN OF
INFECTION“
Different infections have different
epidemiologies and thus require different
methods of control
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In the practical part it is preoccupied with
preventive measures
repressive measures
related to infectious diseases
The 14 steps of an epidemic investigation
1. Confirm the existence of an epidemic.
2. Verify the diagnosis.
3. Develop a case definition.
4. Develop a case report form.
5. Count the cases (i.e., an approximate analysis).
6. Orient the data (i.e., time, place, and person).
7. Analyze the data (e.g., agent, transmission, and host).
8. Develop a hypothesis.
9. Test the hypothesis.
10.Plan and implement control and prevention measures.
11.Evaluate the implemented measures.
12.Establish or improve the public health surveillance.
13.Write a report.
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