Containment Strategies - Book: Communicable
Diseases (Webber)
Chapter 1 - Elements of Communicable Diseases
1.1 What Are Communicable Diseases?
Communicable disease: an illness that is transmitted from a person, animal or inanimate source to
another person either directly, with the assistance of an intermediate host, or by a vector.
Epidemic: the introduction of a new infection or the presence of an illness in excess of normal
expectancy. Any unknown infection will be epidemic when first introduced.
Endemic: constantly present in a geographical area or population group, e.g. malaria is found in
tropical countries and endemic in the adult population.
- With endemic diseases it is children that are particularly vulnerable, so there is a high birth
rate to compensate
Think of communicable diseases in terms of:
- Agent
- Transmission
- Host
- Environment
1.2 The Agent
● Organism (virus, bacteria, protozoa, etc.) or physical or chemical agent (toxin or poison)
○ If it is an organism, it needs to multiply, find a means of transmission and survive
- Relevant in treatment control
- Asexual: a succession of exact or almost exact replicas are produced, so that
any natural selection will act on batches or strains, rather than on
individuals
- Sexual: great scope of variety, this means that natural selection acts on
individuals, and variations of vigour and adaptability occur
, ● Survival: agents have been able to improve their chances of finding a new host or surviving
in the environment by a number of methods
○ Reservoirs and parasite adaptability
- Reservoirs can be humans, animals, vectors or inanimate environment (e.g.
soil, water)
○ Persistence
1.3 Transmission
● Direct: person-to-person contact (e.g. dirty fingers, droplet)
● Human reservoir with intermediate host: for transmission to another human the agent must
undergo developmental stages in an intermediate host
● Animals as intermediate host or reservoir
● Vectors: carries the infection from one host to another (in transmission process or
mechanically)
Zoonoses: communicable diseases where there is an animal reservoir or intermediate host involved
● Domestic: those invited animals that live in close proximity to humans (pets/farm animals)
● Synanthropic: animals that live in close association with humans, but are not invited (rats)
● Exoanthropic: animals that are not in close association with humans (monkeys, bats)
A carrier is a person that can transmit the infective agent but is not manifesting the disease
● Incubating or prodromal carriers: people who are infectious but unaware that they are in the
early stages of the disease
● Asymptomatic carriers who remain well throughout the infection
● Convalescent carriers who continue to be infectious after the clinical disease has passed
1.4 Host factors
● Susceptibility
○ Genetic
○ Age
○ Sex
○ Pregnancy
● Inherent defence mechanisms
○ Physical barriers such as skin, mucous-secreting membranes or acidity of the
stomach
○ Inflammatory response
● Immunity
○ Cellular response: T-lymphocytes
○ Humoral response: B-lymphocytes and antibodies
○ Active immunity follows an infection or vaccination with attenuated organisms →
immune response
○ Passive immunity is the transfer of antibodies from a mother to her child via the
placenta
● Resistance - may be lowered by the following
, ○ Nutrition
○ Trauma and debilitating conditions
○ Multiple infections
○ Immunodeficiency
1.5 The Environment
The transmission cycle used by the agent to reach the host takes place within an environment that
determines the success and severity of the infection.
● Social environment
○ Education
- Complex process, they must be able to understand to such extent to be able
to modify their lives
○ Resources and economics
- Cost-effectiveness programmes
○ Communities and movements
- The type of structure people live in can play an important part in the
diseases they succumb to
- Local migrations from one country to a neighbouring country for trade or to
visit relatives can risk the health of individuals or families
● Physical environment
○ Topography
○ Climate
- Temperature: Temperature not only affects the presence or absence of
disease, but often regulates the amount
- Rainfall: Moderate rainfall creates fresh breeding sites for Anopheles
mosquitoes, but excessive rain can wash out larvae and cause a reduction in
the number of mosquitoes
- Wind
○ Natural disasters
- Natural disasters disrupt the normal pattern of life, destroy water supplies
and provide ideal conditions for epidemics to occur
○ Seasonality (temperature + rainfall): many illnesses show a marked seasonal pattern
○ Climate change: increase in vector-borne and diarrhoeal diseases, malnutrition and
natural disasters
- E.g. flooding will make diarrhoeal diseases more likely and extend the range
of diseases that favour a watery environment
- Increase in temperature has the potential to expand the range of infections
that are normally constrained by temperature
Diseases (Webber)
Chapter 1 - Elements of Communicable Diseases
1.1 What Are Communicable Diseases?
Communicable disease: an illness that is transmitted from a person, animal or inanimate source to
another person either directly, with the assistance of an intermediate host, or by a vector.
Epidemic: the introduction of a new infection or the presence of an illness in excess of normal
expectancy. Any unknown infection will be epidemic when first introduced.
Endemic: constantly present in a geographical area or population group, e.g. malaria is found in
tropical countries and endemic in the adult population.
- With endemic diseases it is children that are particularly vulnerable, so there is a high birth
rate to compensate
Think of communicable diseases in terms of:
- Agent
- Transmission
- Host
- Environment
1.2 The Agent
● Organism (virus, bacteria, protozoa, etc.) or physical or chemical agent (toxin or poison)
○ If it is an organism, it needs to multiply, find a means of transmission and survive
- Relevant in treatment control
- Asexual: a succession of exact or almost exact replicas are produced, so that
any natural selection will act on batches or strains, rather than on
individuals
- Sexual: great scope of variety, this means that natural selection acts on
individuals, and variations of vigour and adaptability occur
, ● Survival: agents have been able to improve their chances of finding a new host or surviving
in the environment by a number of methods
○ Reservoirs and parasite adaptability
- Reservoirs can be humans, animals, vectors or inanimate environment (e.g.
soil, water)
○ Persistence
1.3 Transmission
● Direct: person-to-person contact (e.g. dirty fingers, droplet)
● Human reservoir with intermediate host: for transmission to another human the agent must
undergo developmental stages in an intermediate host
● Animals as intermediate host or reservoir
● Vectors: carries the infection from one host to another (in transmission process or
mechanically)
Zoonoses: communicable diseases where there is an animal reservoir or intermediate host involved
● Domestic: those invited animals that live in close proximity to humans (pets/farm animals)
● Synanthropic: animals that live in close association with humans, but are not invited (rats)
● Exoanthropic: animals that are not in close association with humans (monkeys, bats)
A carrier is a person that can transmit the infective agent but is not manifesting the disease
● Incubating or prodromal carriers: people who are infectious but unaware that they are in the
early stages of the disease
● Asymptomatic carriers who remain well throughout the infection
● Convalescent carriers who continue to be infectious after the clinical disease has passed
1.4 Host factors
● Susceptibility
○ Genetic
○ Age
○ Sex
○ Pregnancy
● Inherent defence mechanisms
○ Physical barriers such as skin, mucous-secreting membranes or acidity of the
stomach
○ Inflammatory response
● Immunity
○ Cellular response: T-lymphocytes
○ Humoral response: B-lymphocytes and antibodies
○ Active immunity follows an infection or vaccination with attenuated organisms →
immune response
○ Passive immunity is the transfer of antibodies from a mother to her child via the
placenta
● Resistance - may be lowered by the following
, ○ Nutrition
○ Trauma and debilitating conditions
○ Multiple infections
○ Immunodeficiency
1.5 The Environment
The transmission cycle used by the agent to reach the host takes place within an environment that
determines the success and severity of the infection.
● Social environment
○ Education
- Complex process, they must be able to understand to such extent to be able
to modify their lives
○ Resources and economics
- Cost-effectiveness programmes
○ Communities and movements
- The type of structure people live in can play an important part in the
diseases they succumb to
- Local migrations from one country to a neighbouring country for trade or to
visit relatives can risk the health of individuals or families
● Physical environment
○ Topography
○ Climate
- Temperature: Temperature not only affects the presence or absence of
disease, but often regulates the amount
- Rainfall: Moderate rainfall creates fresh breeding sites for Anopheles
mosquitoes, but excessive rain can wash out larvae and cause a reduction in
the number of mosquitoes
- Wind
○ Natural disasters
- Natural disasters disrupt the normal pattern of life, destroy water supplies
and provide ideal conditions for epidemics to occur
○ Seasonality (temperature + rainfall): many illnesses show a marked seasonal pattern
○ Climate change: increase in vector-borne and diarrhoeal diseases, malnutrition and
natural disasters
- E.g. flooding will make diarrhoeal diseases more likely and extend the range
of diseases that favour a watery environment
- Increase in temperature has the potential to expand the range of infections
that are normally constrained by temperature
