The epidemiological triad model shows the interaction between an agent, host, and environment in causing disease. The agent is the cause of disease, such as a virus or bacteria. The host is the human or animal susceptible to the disease. Environmental factors allow for disease transmission over time between the agent and host. Breaking the connection between any of these three factors can help prevent and control the spread of disease.

1. Epidemiological
triad
Presented by : Kumar Nyaupane

2. Introduction
• The germ theory of disease has many limitations
.For example it is well known that not everyone
exposed to tuberculosis develops tuberculosis
.Similarly not everyone exposed to beta-
haemolytic streptococci develops acute rheumatic
fever.
• There are other factors relating to host and
environment which are equally important to
determine whether or not disease will occur in
exposed host.

3. Contd….

4. Contd…
• The triangle is based on the communicable
disease model and is useful in showing the
interaction and interdependence of agent,host and
environment and time as used in the investigation
of disease and epidemics
• The agent is cause of disease,the host is an
organism or human or animal that harbous the
disease and environment is surrounding and
condition that allow disesae transmission and time
accounts for incubation period,life expectancy of
host or pathogen and duration or course of illness.

5. Agent factors
1. Biological agent :
: Viruses, Bacteria, Protozoa,parasite,Fungi
2. Nutrient agent :
: Protein, Carbohydrate,fat,water, minerals and
vitamins excess or deficiency.
: Anemia,PEM,Vitamin def., minerals def.

6. Contd….
3. Physical agent :
: exposure to heat,cold,humidity,rediation
electricity,sound
4. Chemical agent :
: Endogenous - Urea,bilirubin,ketones,uric acid
: Exogenous – allergens,metal,fumes,dust
5. Mechanical agent :
: Crushing,tearing,dislocation

7. Host factors
1. Demographic factors
- age, sex, ethnicity
2. Biological characteristics
- Blood group,enzymes,immunology,physiology
3. Socioeconomic factors
- Economic,education,housing,occupation
4. Lifestyle related factors
- Exercise,Smoking,eating pattern,behavors

8. Environmental factors
1. Physical environment
- Air,water,soil,housing,climate,geography
2. Biological environment
- Viruses,fungi,plants,insects,rodents
3. Psychosocial environment
- Habits,beliefs,attitude,religions,customs

9. Disease control and prevention
• The primary mission of epidemiology is to
provide information that results in breaking
one of the leg of triangle thereby disrupting
the connection among agent ,host and
environment and stopping outbreaks.
• Applying the triad model to prevention, to
prevent the disease it should be sufficient to
modify one or more of the factors of the
corner of the triangle.

10. Contd…
• To immunize the host or to develop a
protective environment or to remove agent .
These fall under health protection and hazard
control measures.
• If you can not remove or modify the corners of
the triangle you can prevent disease by
breaking the lines.
• Remove the connection between the agent
and the environment.

11. Contd…
• Example Install window screen to keep
malarial mosquito out of the house.
• Protect the house host from coming in contact
with agent example, use bug spray or
mosquito nets.
• Move the host to a different environment
example leave town.
• This idea is taken up in the source-path-
receiver model in occupational health.