The document discusses epidemiology and the chain of infection. It defines epidemiology as the study of health-related states and events, including disease distribution and control. It then explains the chain of infection, noting that infection only spreads when all the necessary conditions, or links in the chain, are present. These include an infectious agent, a reservoir, means of exiting the reservoir, transmission between hosts, entry through a susceptible host's portal of entry. Breaking any link stops spread. It provides examples of each link for common infectious diseases like tuberculosis.

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EPIDEMIOLOGY

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 Epidemiology is the study of the
distribution and determinants of health-
related states or events (including
disease), and the application of this study
to the control of diseases and other
health problems.
-WHO

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Epidemiology
noun ep·i·de·mi·ol·o·gy ˌe-pə-ˌdē-mē-ˈä-lə-jē, -
ˌde-mē-
medical : the study of how disease spreads and
can be controlled.
- Merriam-Webster Dictionary

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The Chain of Infection
 Infection can only spread when the conditions are
right – we call this set of conditions the 'chain of
infection'.
 Think of the conditions as links in a chain – when all
the links are connected, infection spreads. More
importantly, 'breaking the chain' at any point stops
infection from spreading. Our infection control
processes and procedures all aim to break the chain
of infection.

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Infectious Agents
 Any microorganism that can cause a disease such as
a bacterium, virus, parasite, or fungus.
 Reasons that the organism will cause an infection
are:
virulence (ability to multiply and grow)
invasiveness (ability to enter tissue)
pathogenicity (ability to cause disease)

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Infectious Agents
Bacterium
Virus
Fungus

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Bacteria
Bacteria can survive in virtually all environments,
and their spores are even tougher - being able to
germinate after several years of being dormant.
Though some bacteria can cause food poisoning
and infectious diseases in humans, most are
harmless.
Many bacteria are actually beneficial and are
used in various industrial processes – especially
in the food industry such as the production of
yoghurt and cheeses.

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Tuberculosis

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Virus
 Viruses survive and reproduce inside a single living
cell – plant, animal, or even bacteria.This makes
treating viruses difficult, as many things that will
damage the virus will damage the host cell as
well.
 However most are not very strong if they are outside
a living cell and can only survive for a short time if
they don't find a new host.

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Ebola Virus

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Fungus
 Most fungi prefer to live in moist, warm and dark
environments – it stays alive by absorbing nutrients from
organic matter such as food or soil. However fungal
spores can survive for a long time in a variety of
environments.
 About 200 000 species of organisms belong to the
kingdom Fungi, including yeasts, rusts, molds,
mushrooms, and mildew.
 Fungus can cause such diseases as athlete's foot (Tinea
pedis) and ringworm in humans.

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Athlete’s Foot

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Difference between bacteria and
virus
 Bacteria are single-celled, prokaryotic microorganisms
that exist in abundance in both living hosts and in all
areas of the planet (e.g., soil, water). By their nature,
they can be either "good" (beneficial) or "bad" (harmful)
for the health of plants, humans, and other animals that
come into contact with them.
 A virus is acellular (has no cell structure) and requires a
living host to survive; it causes illness in its host, which
causes an immune response.

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Reservoir
The 'reservoir' is where microbes live and
where microorganisms can survive, thrive
and reproduce.

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Reservoirs
People
Animals
Equipment
Water

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1. People
People are the perfect environment for
microbes, providing warmth, moisture, oxygen,
carbon and food – everything a microbe needs.
Although many microbes are good for our
health, when the wrong microbes enter the
wrong part of the body, we get sick.
2. Animals
Animals provide a perfect environment for
microbes. For example ringworm, ticks and
mosquitos.

18. 3. Equipment
Most microbes can survive for a certain amount of time
outside the human body, so they can and will live on
equipment. Some can last for a long time on surfaces like
table tops, bed linen, doorknobs and furniture.
4. Water
Microbes can live in water in the environment, eg in
puddles, drains and containers.When the conditions are
right, they may multiply and be flushed out by rain to
spread disease. Unclean drinking water supplies and air-
conditioning components can also be a source of
microbes.

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Portal of Exit
A place of exit providing a way for a
microorganism to leave the reservoir.
Respiratory tract
Gastrointestinal tract
Genitourinary tract
Mucous membrane
Skin

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RespiratoryTract
 The respiratory tract includes the mouth, nose, throat
and lungs. Microorganisms can exit the body through
the respiratory tract via coughing, sneezing and
exhaling.
Gastrointestinal Tract
 The gastrointestinal tract includes the stomach, and
small and large intestines. It is the system that takes in
food, digests it and expels the waste. Portals of exit
within the gastrointestinal tract include the oral cavity,
eg vomiting, and the rectum, eg faeces.

21. +  GenitourinaryTract
 The genitourinary tract includes the reproductive system and the
urinary system. Microorganisms can exit the tract via the urethra
(in male and females) and the vagina (in females).
 Mucous Membrane
 Mucous membranes line the respiratory, gastrointestinal and
genitourinary tracts and the eyes. Microorganisms can exit the
body through secretions from the membranes such as tears from
the tear duct near the eyes, and nasal secretions from the nose.
 Skin
 The skin provides a barrier to infection when it is intact. If skin is
broken it can become a portal of exit. In case of a wound,
secretions from the wound (ie blood or serous fluid) can spread
the microorganism from the portal of exit.

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Mode of Transmission
The way in which the organism moves or is
carried from one place to another.
Contact
Droplets
Airborne

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Contact
Infection can be transmitted by one person
touching another person or their bodily fluids.
For example, a healthcare worker could carry
infection on their hands and pass it on from
one person to another.
Infection can also pass from a person to an
object, so if an infected person touches an
object – eg a door knob, bed rail, pen or light
switch – another person who touches it may
transfer the infection to their own body.

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Droplets
Droplets are very small drops of liquid that can
exit the body during coughing, sneezing,
sexual intercourse or during medical
procedures, carrying microorganisms.
Airborne
Pathogens in dust or tiny droplets can hang in
the air for a long time, eventually settling on
surfaces or being inhaled.

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Portal of entry
An opening allowing the microorganism to enter
the host.
Respiratory tract
Gastrointestinal tract
Genitourinary tract
Mucous membrane
Skin

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Respiratory tract
 It provides a warm, moist entry point for airborne
pathogens that are inhaled.
Gastrointestinal tract
 Pathogens can enter this tract on contaminated
food or fluids.
Genitourinary tract
 Pathogens can enter the tract by via the urethra,
and the vagina (in females). Urinary catheters can
also provide a portal of entry.

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Mucous membrane
 The mucous membranes are designed to trap
pathogens and help protect the body against
pathogens, if the mucous membrane fails or is
broken – eg by an ulcer or wound – then microbes
can enter.
Skin
 When the skin is healthy and intact it protects us.
However if the skin is broken by cracks, bites,
scrapes, cuts, wounds or ulcers pathogens can
enter. Punctures produced by medical treatments
like injections, drainage and intravenous fluid tubes
can also provide a portal of entry.

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Susceptible host
The human body has natural defenses to prevent
pathogens entering via the portals of entry
described above, but some people get sick
anyway. People who are susceptible hosts lack
the immunity to overcome invasion by
microorganisms.

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Susceptible hosts
Cancer Patients
Newborns
Elderly Patients
Surgical Patients
Burns Patients
Diabetes Mellitus

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Cancer Patients
 Both cancer and cancer treatments can weaken the
immune system.
Newborns
 A baby's immune system takes a few months to
become fully developed, so they are vulnerable to
infection while they're very young.
Elderly Patients
 As people age their immune systems change, so the
elderly may fight infection less quickly and less
effectively.

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Surgical Patients
 Surgery and wounds cut the skin, which gives
pathogens a portal of entry. Surgery and anaesthetic
both have an impact on the immune system.
Burns Patient
 Burns injuries alter the immune system which makes
patients more vulnerable to infection. Open wounds on
the skin give pathogens a portal of entry.
Diabetes Mellitus
 Diabetes mellitus weakens the immune system,
therefore diabetes patients are more prone to
infection.

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BREAKING THE CHAIN
Breaking the chain of infection at any point
stops infection from spreading, however you
should always aim to break the chain of infection
as early as possible.

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Chain of Infection of Tuberculosis
Infectious Disease Agent
 Mycobacterium tuberculosis

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Reservoir
The only reservoir forTB is the human.
Portal of Exit
TB's portal of exit is via the mouth and
nose.When someone withTB sneezes
or coughs, they release large numbers
of theTB microbacterium.

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Mode ofTransmission
The mode of transmission is the cough or
sneeze that releasesTB bacterium into the air.
It can then be inhaled by another person in the
room.
Susceptible Hosts
Humans with weak immune system.