Containment strategies – Webber chapter 1 – 6
Chapter 1 – Elements of Communicable Diseases
• What are Communicable Diseases
A communicable disease is an illness that is transmitted from a person, animal or inanimate
source to another person either directly, with the assistance of a vector or by other means.
Communicable Non-communicable
Endemic Acute
Epidemic Chronic
More common in developing countries More common in the developed world
More frequent as people live longer
→ this difference was quite clear-cut. When it was an organism that was transmitted, the disease
was communicable; otherwise the disease was classified as non-communicable. However, this
is becoming less well-defined as new suspect organisms are discovered or diseases, by their
nature, suggest a communicable origin.
Epidemic – devastate whole populations. Populations have to start again from the survivors to
recover their former strength → essentially young and growing populations
Endemic – children are particularly vulnerable, so there is a high birth rate to compensate.
The key to any communicable disease is to think of it in terms of agent, transmission, host and
environment – there needs to be a causative agent, which requires a means of transmission from
one host to another, but the outcome of infection will be influenced by the environment in which
the disease is transmitted.
• The Agent
Can be an organism (virus, bacteria, fungus, protozoan, helminth), a physical or a chemical
agent (toxin or poison). If the agent is an organism, it needs to multiply and find a means of
transmission and survival.
Multiplication:
Sexual Asexual
Offers great scope of variety, both within the A succession of exact or almost exact replicas
cells of the single organism and from one are produces, so that any natural selection
organism to the other. Natural selection acts will act on batches or strains rather than on
on individuals and variations occur individuals
Certain individuals may succeed while others Strain of the organism is either successful or
may not unsuccessful in invading the host
Both male and female adults must meet Only one organism of the asexual parasite
before reproduction requires to be transmitted
→ relevant in treatment and control. If a treatment is successful in destroying an asexually
reproducing organism, it will also be successful against all individuals (unless a mutation
occurs). in contrast, with sexual treatment some individuals will succumb to treatment, while
others will not. However, having two sexes can be a disadvantage for the organism in that
methods of control can be devised to attack only one of the sexes or prevent them from meeting.
, Agents survive by finding a suitable host within a certain period of time. They improve their
chances of finding a new host, or prolonging this period by different methods:
Reservoirs When the parasite first attacks a new species, the host attempts to eliminate
and parasite (severe reaction). Then adaptation can occur, wo that the reaction decreases
adaptability and the adaptability of the parasite increases. The host acts as a reservoir in
which the parasite can colonize different species and attack new hosts
Persistence Development of special stages that resist destruction in an adverse
environment. For example, bacteria can persist in the environment by the
development of spores as with anthrax and tetanus bacilli.
Latency Developmental stage in the environment that is not infective to a new host.
Allows the parasite time for suitable conditions to develop before changing
into the infective form
Illness occurs when enough agents survive to infect a new host. The severity is determined by
its toxicity and virulence.
- Toxic reaction due to foreign proteins they consist or produce (respiratory/reproductive)
- Production of toxins when they grow in food, causing illness at a distance
- Influenza > as an infection progresses in a community, virulence can increase or
decrease due to its passage through several individuals.
The number of organisms excreted can vary considerably due to the type of infection or the
stage of the disease. for each infectious agent, a minimum number of organisms (the infective
dose) is required to overcome the defences of the host and cause the disease – in most infections,
once this number is surpassed, the severity of the disease is the same whether a few or large
number of organisms is introduced. While in others, there is a correlation between dose and
severity of the illness.
- Infections with a low infective dose can spread by person-to-person contact
- When a high infective dose is required (cholera), the improvement of water quality and
the reduction of pathogens in the sewage will be beneficial to the community
Chapter 1 – Elements of Communicable Diseases
• What are Communicable Diseases
A communicable disease is an illness that is transmitted from a person, animal or inanimate
source to another person either directly, with the assistance of a vector or by other means.
Communicable Non-communicable
Endemic Acute
Epidemic Chronic
More common in developing countries More common in the developed world
More frequent as people live longer
→ this difference was quite clear-cut. When it was an organism that was transmitted, the disease
was communicable; otherwise the disease was classified as non-communicable. However, this
is becoming less well-defined as new suspect organisms are discovered or diseases, by their
nature, suggest a communicable origin.
Epidemic – devastate whole populations. Populations have to start again from the survivors to
recover their former strength → essentially young and growing populations
Endemic – children are particularly vulnerable, so there is a high birth rate to compensate.
The key to any communicable disease is to think of it in terms of agent, transmission, host and
environment – there needs to be a causative agent, which requires a means of transmission from
one host to another, but the outcome of infection will be influenced by the environment in which
the disease is transmitted.
• The Agent
Can be an organism (virus, bacteria, fungus, protozoan, helminth), a physical or a chemical
agent (toxin or poison). If the agent is an organism, it needs to multiply and find a means of
transmission and survival.
Multiplication:
Sexual Asexual
Offers great scope of variety, both within the A succession of exact or almost exact replicas
cells of the single organism and from one are produces, so that any natural selection
organism to the other. Natural selection acts will act on batches or strains rather than on
on individuals and variations occur individuals
Certain individuals may succeed while others Strain of the organism is either successful or
may not unsuccessful in invading the host
Both male and female adults must meet Only one organism of the asexual parasite
before reproduction requires to be transmitted
→ relevant in treatment and control. If a treatment is successful in destroying an asexually
reproducing organism, it will also be successful against all individuals (unless a mutation
occurs). in contrast, with sexual treatment some individuals will succumb to treatment, while
others will not. However, having two sexes can be a disadvantage for the organism in that
methods of control can be devised to attack only one of the sexes or prevent them from meeting.
, Agents survive by finding a suitable host within a certain period of time. They improve their
chances of finding a new host, or prolonging this period by different methods:
Reservoirs When the parasite first attacks a new species, the host attempts to eliminate
and parasite (severe reaction). Then adaptation can occur, wo that the reaction decreases
adaptability and the adaptability of the parasite increases. The host acts as a reservoir in
which the parasite can colonize different species and attack new hosts
Persistence Development of special stages that resist destruction in an adverse
environment. For example, bacteria can persist in the environment by the
development of spores as with anthrax and tetanus bacilli.
Latency Developmental stage in the environment that is not infective to a new host.
Allows the parasite time for suitable conditions to develop before changing
into the infective form
Illness occurs when enough agents survive to infect a new host. The severity is determined by
its toxicity and virulence.
- Toxic reaction due to foreign proteins they consist or produce (respiratory/reproductive)
- Production of toxins when they grow in food, causing illness at a distance
- Influenza > as an infection progresses in a community, virulence can increase or
decrease due to its passage through several individuals.
The number of organisms excreted can vary considerably due to the type of infection or the
stage of the disease. for each infectious agent, a minimum number of organisms (the infective
dose) is required to overcome the defences of the host and cause the disease – in most infections,
once this number is surpassed, the severity of the disease is the same whether a few or large
number of organisms is introduced. While in others, there is a correlation between dose and
severity of the illness.
- Infections with a low infective dose can spread by person-to-person contact
- When a high infective dose is required (cholera), the improvement of water quality and
the reduction of pathogens in the sewage will be beneficial to the community
