This document provides an overview of essential epidemiology concepts. It defines communicable diseases as those caused by transmission of an infectious agent from person to person, either directly or indirectly through vectors or vehicles. Epidemiology studies the interaction between infectious agents, hosts, and the environment in disease outbreaks. It has helped increase ability to control spread of diseases through surveillance, prevention and treatment. Emerging and reemerging diseases continue to burden health systems, particularly in low-income countries. Epidemics are defined as excess disease cases in a community, while endemic diseases have relatively stable high prevalence in a geographic area or population group.

1. ESSENTIAL EPIDEMIOLOGY
By:
WAHAB ALI WAZIR
MPhil Scholar
2nd Semester
Department of Zoology
Course Instructor
DR. KALIM ULLAH
Assistant Professor
Department of Zoology
Department Of Zoology
Kohat University Of Science &Technology,Kohat-26000
Khyber Pakhtunkhwa, Pakistan

2. Introduction
 A communicable (or infectious) disease is one caused by
transmission of a specific pathogenic agent to a
susceptible host. Infectious agents may be transmitted to
humans either:
 directly, from other infected humans or animals, or
indirectly, through vectors, airborne particles or vehicles.
 Vectors are insects or other animals that carry the
infectious agent from person to person. Vehicles are
contaminated objects or elements of the environment
(such as clothes, cutlery, water, milk, food, blood, plasma,
parenteral solutions or surgical instruments).

3. Conti…
Contagious diseases are those that can be spread (contagious
literally means “by touch”) between humans without an
intervening vector or vehicle.
Malaria is therefore both communicable and contagious.
Some pathogens cause disease not only through infection but
through the toxic effect of chemical compounds that these produce.
For example, Staphylococcus aureus is a bacteria that can infect
humans directly, but staphylococcal food poisoning is caused by
ingestion of food contaminated with a toxin that the bacteria
produces.

4. Role of epidemiology
 Epidemiology developed from the study of outbreaks of
communicable disease and of the interaction between agents,
hosts, vectors and reservoirs.
 The ability to describe the circumstances that tend to spark
epidemics in human populations – war, migration, famine and
natural disasters
 It has increased human ability to control the spread of
communicable disease through surveillance, prevention,
quarantine and treatment.

5. The burden of communicable disease
 The estimated global burden of communicable diseases –
dominated by HIV/AIDS, tuberculosis and malaria – is shown
in Box 7.1.
 Emerging diseases such as viral haemorrhagic fevers, new
variant Creutzfeld-Jakob disease and severe acute respiratory
syndrome (SARS)
 as well as reemerging diseases including diphtheria, yellow
fever, anthrax, plague, dengue and influenza place a large and
unpredictable burden on health systems, particularly in low-
income countries.

6. Conti…

7. Threats to human security and health
systems
 Communicable diseases pose an acute threat to individual
health and have the potential to threaten collective human
security.
 While low-income countries continue to deal with the problems
of communicable diseases, deaths due to chronic diseases are
rapidly increasing, especially in urban settings.
 Although high-income countries have proportionally less
communicable disease mortality, these countries still bear the
costs of high morbidity from certain communicable diseases.
 For example, in high-income countries, upper respiratory tract
infections cause significant mortality only at the extremes of
age (in children and elderly people).

8. However, the associated morbidity is substantial, and affects
all age-groups (Figure 7.2).
Conti…

9.  Using epidemiological methods to investigate and control
communicable disease is still a challenge for the health profession.
Investigation must be done quickly and often with limited
resources.
 The consequences of a successful investigation are rewarding, but
failure to act effectively can be damaging.
 In the AIDS pandemic, 25 years of epidemiological studies have
helped to characterize the agent, modes of transmission and
effective means of prevention.
 However, despite this knowledge, the estimated global prevalence
of HIV in 2006 was 38.6 million cases, with 3 million deaths per
year.
Conti…

10. Epidemics
 Epidemics are defined as the occurrence of cases in excess of
what is normally expected in a community or region.
 When describing an epidemic, the time period, geographical
region and particulars of the population in which the cases
occur must be specified.
 The number of cases needed to define an epidemic varies
according to the agent, the size, type and susceptibility of
population exposed, and the time and place of occurrence..

11.  The identification of an epidemic also depends on the usual
frequency of the disease in the area among the specified
population during the same season of the year very small number
of cases of a disease not previously recognized in an area,
 but associated in time and place, may be sufficient to constitute an
epidemic.
 For example, the first report on the syndrome that became known
as AIDS concerned only four cases of Pneumocystis carinii
pneumonia in young homosexual men.
 Previously this disease had been seen only in patients with
compromised immune systems.
Conti…

12.  The rapid development of the epidemic of Kaposi sarcoma, another
manifestation of AIDS,
 in New York is shown in Figure 7.3: 2 cases occurred in 1977 and
1978 and by 1982 there were 88 cases.
Conti…

13. Endemic diseases
 Communicable diseases are termed endemic when they have a
relatively stable pattern of occurrence in a given geographical
area or population group at relatively high prevalence and
incidence.
 Endemic diseases such as malaria are among the major health
problems in low-income tropical countries.
 If conditions change in the host, the agent or the environment,
an endemic disease may become epidemic.
 For example, improved smallpox control in Europe was
reversed during the First World War (Table 7.1).

14. Conti…

15.  The HIV epidemic is an example of an infectious disease that
has become endemic in many areas, while in other areas it still
causes epidemics in previously unexposed populations.
 In the cases of malaria and dengue fever, where mosquitoes are
the vectors, the endemic areas are constrained by climate.
 If the area is too cold or dry and the vector cannot survive or
reproduce, the disease does not become endemic.
 Global warming is changing the climate in some parts of the
world in ways that increase the size of the endemic areas, and
these vector-borne diseases are spreading to new areas
Conti…