Summary Communicable diseases
Chapter 1 Elements of Communicable diseases
A communicable disease 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. They include
parasitic diseases, infections transmitted by a vector, zoonoses and all transmissible diseases.
Epidemic is 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 is a constantly present disease in a geographical area or population group.
Communicable diseases are dependent on the person being susceptible to infection. They are
found in conditions that encourage transmission. Communicable diseases could be reduced to
manageable problems if enough resources could be spent on them.
The key to any communicable disease is to think of it in terms of agent, transmission, host, and
environment.
Agent this can be an organism or a physical or chemical agent. If it is an organism, the agent
needs to multiply, find a means of transmission and survive.
- Multiplication:
o Asexual multiplication: 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. Invading the host can either be successful or unsuccessful.
o Sexual reproduction: offers a great variety, within the cell and in different
organisms. Natural selection acts on individuals, variations of vigor and adaptability
occur. Certain individuals will successfully invade the host, whereas others will not.
- Survival:
o A reservoir is a suitable place for storing agents of infection. When a parasite
attacks a new species, the host will attempt to eliminate it, resulting in a severe
reaction. In time, adaptation of the agent will occur, and the reaction of the host
diminishes and the adaptability increases. The parasite is then able to live in the
host without making it ill. The host will then act as a reservoir. The host acts as a
reservoir to infect other hosts.
, o Persistence: a bacterium is able to persist in an environment by the development of
spores. Then antiseptics or sufficient heat is necessary to destruct the organism.
o Latency: this is when an organism is in a developmental stage when it is not
infective. This allows the parasite to wait until suitable conditions develop before
changing into an infective form.
- Effect of the agent:
o If enough agents survive to infect the host, they will produce illness. Severity is
determined by virulence and toxicity of the agent. Infectious agents produce a toxic
reaction due to the foreign proteins they produce in their respiratory and
reproductive processes.
- Excreted load and infective dose
o The number of organisms excreted can vary according to the type of infection or
the stage of the disease. Infections with low infective dose can spread from person-
to-person contact, and therefore provision of safe water supply or sanitation have
little to no effect. With infections with high infective dose, improving water quality
and the reduction of pathogens will be beneficial.
Transmission
- Direct: includes person-to-person contact, through dirty fingers, food and water. It can also
occur through droplet infection in respiratory diseases. Autoinfection can occur when you
infect yourself by e.g., anal scratching or infection of skin abrasions with bacteria from nose
picking.
- Human reservoir with intermediate host: some parasites live in humans, but for
transmission to another human the parasite must undergo developmental stages.
- Animals as intermediate host or reservoir : the parasite must develop first in the animal
before it can infect humans, or animals can act as reservoirs.
- Vectors: a vector carries the infection from one host to another either as part of the
transmission process or mechanically
o Direct insect to human as in malaria
o Insect to animal with humans entering the cycle as an abnormal host, in bubonic
plague
o Insect to animal, from which it is transmitted to other humans by the same or
another insect vector. E.g., yellow fever.
- Zoonosis: these are infections that are naturally transmitted between vertebrate animals
and humans. These can be grouped according to the intimacy of the animal to the human
o Domestic: those invited animals that live in close proximity to humans
o Synanthropic: animals that live in close association with humans, but are not
invited, rats
o Exoanthropic: animals that are not in close association with humans
- Plants: vegetable material that is eaten by the host can serve as a method of transmission.
- Carriers and subclinical transmission: a carrier is a person that can transmit the infective
agent, but is not manifesting the disease.
o Incubating or prodromal carriers: who are infectious but unaware that they are in
the early stages of the disease
o Asymptomatic: who remain well throughout the infection
o Convalescent: who continue to be infectious after the clinical disease has passed
The carrier state can either be transient or chronic
,Host factors the success of an invasion into a host is dependent on:
- Susceptibility
- Inherent defense mechanism
- Immunity: either cellular (T-lymphocytes) or humoral (B-lymphocytes)
- Resistance
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. This can be divided into
the social environment and the physical environment.
The social environment is modified by education and resources and altered by movements of
communities or individuals.
Physical environment is affected by the nature of surroundings, seasonality and climate change.
GIS = geographical information systems. This is the use of complex computer software to analyze a
range of coordinates to identify links between them. Especially helpful in vector-borne diseases.
Chapter 2 Communicable disease theory
Number of new cases of the infection are dependent on the number of infectious persons within a
susceptible population and the degree of contact between them.
Force of infection: number of infectious individuals X transmission rate
Number of newly infected individuals: force of infection X number of susceptible individuals in
the population
If the susceptible population is large enough, (the case when there is little to no immunity), the
force of infection is constant newly infected individuals will be continued to be produced. If
there is a long period of infectiousness, infecting over a longer period of time, an example are
parasites, such as hookworm. In the case of short period of infectiousness, a large number of
people might get infected, an example of this is measles, since this produces immunity.
Endemic: a continuous presence of an infection in the community, and is described with incidence
and prevalence measures.
Epidemics: an excess of cases in the community from that normally expected. When an endemic
becomes epidemic, is dependent of exceeding the epidemic threshold. This can either be the
upper limit of cases expected at that particular time, an excess mortality, or combination.
Characteristics of an epidemic are:
- Latent period: the time interval from initial infection until the start of infectiousness
- Incubation period: the time interval from initial infection until the onset of clinical disease.
This period has a range from the shortest incubation time to longest.
- Period of communicability: the period during which an individual is infectious. The
infectious period can start before the disease process commences or after.
, Transmissions increase in a susceptible population up to a peak and then tail off over a longer
period. The infecting dose, portal of entry, immune response of the host and a number of other
factors modify the normal distribution to extend the tail of the graph. By using a log timescale, this
skewed curve can be converted to a normal distribution and the mean incubation period
measured.
An epidemic can either be a common-source epidemic or propagated-source epidemic:
- Common-source:
o Point-source epidemic: resulting from a single exposure, such as food poisoning
Epidemic curve
Incubation period of the disease, and
Time of infection
o Extended epidemic: resulting from repeated exposures over a length of time
(contaminated well)
The time of infection can be deduced by measuring back in time from the
first case to the maximum and minimum incubation periods of the disease.
Chapter 1 Elements of Communicable diseases
A communicable disease 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. They include
parasitic diseases, infections transmitted by a vector, zoonoses and all transmissible diseases.
Epidemic is 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 is a constantly present disease in a geographical area or population group.
Communicable diseases are dependent on the person being susceptible to infection. They are
found in conditions that encourage transmission. Communicable diseases could be reduced to
manageable problems if enough resources could be spent on them.
The key to any communicable disease is to think of it in terms of agent, transmission, host, and
environment.
Agent this can be an organism or a physical or chemical agent. If it is an organism, the agent
needs to multiply, find a means of transmission and survive.
- Multiplication:
o Asexual multiplication: 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. Invading the host can either be successful or unsuccessful.
o Sexual reproduction: offers a great variety, within the cell and in different
organisms. Natural selection acts on individuals, variations of vigor and adaptability
occur. Certain individuals will successfully invade the host, whereas others will not.
- Survival:
o A reservoir is a suitable place for storing agents of infection. When a parasite
attacks a new species, the host will attempt to eliminate it, resulting in a severe
reaction. In time, adaptation of the agent will occur, and the reaction of the host
diminishes and the adaptability increases. The parasite is then able to live in the
host without making it ill. The host will then act as a reservoir. The host acts as a
reservoir to infect other hosts.
, o Persistence: a bacterium is able to persist in an environment by the development of
spores. Then antiseptics or sufficient heat is necessary to destruct the organism.
o Latency: this is when an organism is in a developmental stage when it is not
infective. This allows the parasite to wait until suitable conditions develop before
changing into an infective form.
- Effect of the agent:
o If enough agents survive to infect the host, they will produce illness. Severity is
determined by virulence and toxicity of the agent. Infectious agents produce a toxic
reaction due to the foreign proteins they produce in their respiratory and
reproductive processes.
- Excreted load and infective dose
o The number of organisms excreted can vary according to the type of infection or
the stage of the disease. Infections with low infective dose can spread from person-
to-person contact, and therefore provision of safe water supply or sanitation have
little to no effect. With infections with high infective dose, improving water quality
and the reduction of pathogens will be beneficial.
Transmission
- Direct: includes person-to-person contact, through dirty fingers, food and water. It can also
occur through droplet infection in respiratory diseases. Autoinfection can occur when you
infect yourself by e.g., anal scratching or infection of skin abrasions with bacteria from nose
picking.
- Human reservoir with intermediate host: some parasites live in humans, but for
transmission to another human the parasite must undergo developmental stages.
- Animals as intermediate host or reservoir : the parasite must develop first in the animal
before it can infect humans, or animals can act as reservoirs.
- Vectors: a vector carries the infection from one host to another either as part of the
transmission process or mechanically
o Direct insect to human as in malaria
o Insect to animal with humans entering the cycle as an abnormal host, in bubonic
plague
o Insect to animal, from which it is transmitted to other humans by the same or
another insect vector. E.g., yellow fever.
- Zoonosis: these are infections that are naturally transmitted between vertebrate animals
and humans. These can be grouped according to the intimacy of the animal to the human
o Domestic: those invited animals that live in close proximity to humans
o Synanthropic: animals that live in close association with humans, but are not
invited, rats
o Exoanthropic: animals that are not in close association with humans
- Plants: vegetable material that is eaten by the host can serve as a method of transmission.
- Carriers and subclinical transmission: a carrier is a person that can transmit the infective
agent, but is not manifesting the disease.
o Incubating or prodromal carriers: who are infectious but unaware that they are in
the early stages of the disease
o Asymptomatic: who remain well throughout the infection
o Convalescent: who continue to be infectious after the clinical disease has passed
The carrier state can either be transient or chronic
,Host factors the success of an invasion into a host is dependent on:
- Susceptibility
- Inherent defense mechanism
- Immunity: either cellular (T-lymphocytes) or humoral (B-lymphocytes)
- Resistance
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. This can be divided into
the social environment and the physical environment.
The social environment is modified by education and resources and altered by movements of
communities or individuals.
Physical environment is affected by the nature of surroundings, seasonality and climate change.
GIS = geographical information systems. This is the use of complex computer software to analyze a
range of coordinates to identify links between them. Especially helpful in vector-borne diseases.
Chapter 2 Communicable disease theory
Number of new cases of the infection are dependent on the number of infectious persons within a
susceptible population and the degree of contact between them.
Force of infection: number of infectious individuals X transmission rate
Number of newly infected individuals: force of infection X number of susceptible individuals in
the population
If the susceptible population is large enough, (the case when there is little to no immunity), the
force of infection is constant newly infected individuals will be continued to be produced. If
there is a long period of infectiousness, infecting over a longer period of time, an example are
parasites, such as hookworm. In the case of short period of infectiousness, a large number of
people might get infected, an example of this is measles, since this produces immunity.
Endemic: a continuous presence of an infection in the community, and is described with incidence
and prevalence measures.
Epidemics: an excess of cases in the community from that normally expected. When an endemic
becomes epidemic, is dependent of exceeding the epidemic threshold. This can either be the
upper limit of cases expected at that particular time, an excess mortality, or combination.
Characteristics of an epidemic are:
- Latent period: the time interval from initial infection until the start of infectiousness
- Incubation period: the time interval from initial infection until the onset of clinical disease.
This period has a range from the shortest incubation time to longest.
- Period of communicability: the period during which an individual is infectious. The
infectious period can start before the disease process commences or after.
, Transmissions increase in a susceptible population up to a peak and then tail off over a longer
period. The infecting dose, portal of entry, immune response of the host and a number of other
factors modify the normal distribution to extend the tail of the graph. By using a log timescale, this
skewed curve can be converted to a normal distribution and the mean incubation period
measured.
An epidemic can either be a common-source epidemic or propagated-source epidemic:
- Common-source:
o Point-source epidemic: resulting from a single exposure, such as food poisoning
Epidemic curve
Incubation period of the disease, and
Time of infection
o Extended epidemic: resulting from repeated exposures over a length of time
(contaminated well)
The time of infection can be deduced by measuring back in time from the
first case to the maximum and minimum incubation periods of the disease.
