SUMMARY OF “COMMUNICABLE DISEASES”
,1 Elements of Communicable Diseases
1.1 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 an
intermediate host, or by a vector. Communicable diseases present in an
epidemic or endemic form. An epidemic is the introduction of a new infection or
the presence of an illness in excess of normal expectancy. An endemic disease is
constantly present in a geographical area or population group.
Communicable diseases are dependent on the person being susceptible to
infection. They are found particularly in conditions that encourage transmission,
such as overcrowding or poor hygiene, so are more common in developing
countries. Epidemic diseases devastate whole populations. Populations then
have to start again from the survivors to recover their former strength. With
endemic diseases it is children that are particularly vulnerable, so there is a high
birth rate to compensate.
This division between the developed and developing world is purely artificial
where diseases are concerned. A tropical environment is more favourable to
many diseases than the cooler temperate regions, but even here such tropical
diseases as malaria were once common in Europe. There is nothing new or
different about these artificially divided parts of the world except for the
resources that each is able to devote to the improvement of their health.
Communicable diseases could be reduced to manageable problems if enough
resources, both in financial and educational terms, could be spent on them.
The difference between communicable and non-communicable diseases used to
be quite clear-cut. Where there was an organism that was transmitted, it was
communicable; otherwise the disease was classified as non-communicable.
However, this strict boundary is becoming less well defined as new suspect
organisms are discovered, or diseases, by their very nature, suggest a
communicable origin. For example, certain cancers can be caused by
transmittable viruses.
While avoiding infection is normally the best strategy, our obsession with
cleanliness might be responsible for the increase in allergies, type 1 diabetes,
inflammatory bowel disorders and multiple sclerosis in developed countries.
These are found less commonly in developing countries where conditions of
hygiene are poor and were rare in developed countries in the early part of the
last century. It is thought that our obsession with cleanliness and using
antibacterial products prevents minor infections that stimulate the immune
system. The use of antibiotics, especially in children in the first 3 years of life, is
an important cause. A little bit of dirt is good for us, and especially for young
children when they are developing their immune response.
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 and the response of the host.
2
,1.2 The Agent
The agent can be an organism (virus, prion, bacteria, rickettsia, protozoa,
helminth, fungus or arthropod) or a physical or chemical agent (toxin or poison).
If it is an organism, the agent needs to multiply, find a means of transmission
and survive.
1.2.1 Multiplication
Two methods of multiplication occur, sexual and asexual reproduction, which
have different advantages. In asexual reproduction 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. By contrast, sexual reproduction
offers great scope for variety, both within the cells of the single organism and
from one organism to another. This means that natural selection acts on
individuals, and variations of vigour and adaptability occur.
There are different consequences of these two methods of reproduction. With
asexual organisms the strain of the organism is either successful or unsuccessful
in invading the host, whereas in sexually reproducing organisms, certain
individuals will succeed while others will not. In continuing its existence, only one
organism of the asexual parasite requires to be transmitted, whereas with the
sexually reproducing parasite, both male and female adults must meet before
reproduction can take place.
Whether the organism reproduces sexually or asexually is relevant in treatment
and control. If a treatment is successful at destroying an asexually reproducing
organism, then it will be successful against all the other individuals of that strain
of the organism – unless a mutation occurs. In contrast, sexual reproduction
produces individuals of different vigour, meaning that 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 which
3
, attack only one of the sexes, or designed to reduce the chance of individuals of
each sex from meeting.
1.2.2 Survival
Parasitic agents survive by finding a suitable host within a certain time. They
have been able to improve their chances of finding a new host or surviving in the
environment by a number of different methods.
Reservoirs and parasite adaptability
A reservoir describes a suitable place for storing agents of infection. Once an
agent invades a host, there is normally a latent or waiting period while sufficient
organisms are produced before the main attack is mounted. If the host survives
but is not able to eliminate the invading organism, then the organism can
continue to live and reproduce in the host.
The relationship between the parasite and host is one of continual challenge,
what has been termed a biological arms race. When the parasite first attacks a
new species, the host attempts to eliminate it, resulting in a severe reaction. In
time, adaptation can occur, so that the reaction of the host diminishes and the
adaptability of the parasite increases. The parasite is then able to live in the host
with few ill effects forming an established population, and continuing with
minimal reaction from the host. The host then acts as a reservoir from which
parasites attack new hosts of the same species or attempt to colonize different
species.
Reservoirs can be humans, animals, vectors or the inanimate environment (e.g.
soil, water). Intermediaries may be required before the final reservoir host is
colonized. However, it is always within the parasite’s interest to improve its
reproductive capability, so if a new mutation arises which is beneficial to this
end, then this will be selected, generally to the host’s disadvantage.
The adaptability of parasites to their human hosts might even have advantages
for us. It is a strange irony that it may be preferable to actually introduce (rather
than control) these parasites to combat some allergic reactions.
Persistence
Another mechanism used by parasites to survive is the development of special
stages that resist destruction in an adverse environment. Bacteria can persist in
the environment by the development of spores, as with anthrax and tetanus
bacilli. Destruction of persistent organisms requires the use of antiseptics or
sufficient heat for a prolonged period.
Latency
The production by the organism of a developmental stage in the environment
that is not infective to a new host is called latency. This allows the parasite to
wait until suitable conditions develop before changing into the infective form.
1.2.3 The effect of the agent
If enough agents survive to infect a new host, they will produce illness, the
severity of which is determined by their toxicity and virulence. Infectious agents
produce a toxic reaction due to the foreign proteins they consist of or produce in
4
,1 Elements of Communicable Diseases
1.1 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 an
intermediate host, or by a vector. Communicable diseases present in an
epidemic or endemic form. An epidemic is the introduction of a new infection or
the presence of an illness in excess of normal expectancy. An endemic disease is
constantly present in a geographical area or population group.
Communicable diseases are dependent on the person being susceptible to
infection. They are found particularly in conditions that encourage transmission,
such as overcrowding or poor hygiene, so are more common in developing
countries. Epidemic diseases devastate whole populations. Populations then
have to start again from the survivors to recover their former strength. With
endemic diseases it is children that are particularly vulnerable, so there is a high
birth rate to compensate.
This division between the developed and developing world is purely artificial
where diseases are concerned. A tropical environment is more favourable to
many diseases than the cooler temperate regions, but even here such tropical
diseases as malaria were once common in Europe. There is nothing new or
different about these artificially divided parts of the world except for the
resources that each is able to devote to the improvement of their health.
Communicable diseases could be reduced to manageable problems if enough
resources, both in financial and educational terms, could be spent on them.
The difference between communicable and non-communicable diseases used to
be quite clear-cut. Where there was an organism that was transmitted, it was
communicable; otherwise the disease was classified as non-communicable.
However, this strict boundary is becoming less well defined as new suspect
organisms are discovered, or diseases, by their very nature, suggest a
communicable origin. For example, certain cancers can be caused by
transmittable viruses.
While avoiding infection is normally the best strategy, our obsession with
cleanliness might be responsible for the increase in allergies, type 1 diabetes,
inflammatory bowel disorders and multiple sclerosis in developed countries.
These are found less commonly in developing countries where conditions of
hygiene are poor and were rare in developed countries in the early part of the
last century. It is thought that our obsession with cleanliness and using
antibacterial products prevents minor infections that stimulate the immune
system. The use of antibiotics, especially in children in the first 3 years of life, is
an important cause. A little bit of dirt is good for us, and especially for young
children when they are developing their immune response.
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 and the response of the host.
2
,1.2 The Agent
The agent can be an organism (virus, prion, bacteria, rickettsia, protozoa,
helminth, fungus or arthropod) or a physical or chemical agent (toxin or poison).
If it is an organism, the agent needs to multiply, find a means of transmission
and survive.
1.2.1 Multiplication
Two methods of multiplication occur, sexual and asexual reproduction, which
have different advantages. In asexual reproduction 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. By contrast, sexual reproduction
offers great scope for variety, both within the cells of the single organism and
from one organism to another. This means that natural selection acts on
individuals, and variations of vigour and adaptability occur.
There are different consequences of these two methods of reproduction. With
asexual organisms the strain of the organism is either successful or unsuccessful
in invading the host, whereas in sexually reproducing organisms, certain
individuals will succeed while others will not. In continuing its existence, only one
organism of the asexual parasite requires to be transmitted, whereas with the
sexually reproducing parasite, both male and female adults must meet before
reproduction can take place.
Whether the organism reproduces sexually or asexually is relevant in treatment
and control. If a treatment is successful at destroying an asexually reproducing
organism, then it will be successful against all the other individuals of that strain
of the organism – unless a mutation occurs. In contrast, sexual reproduction
produces individuals of different vigour, meaning that 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 which
3
, attack only one of the sexes, or designed to reduce the chance of individuals of
each sex from meeting.
1.2.2 Survival
Parasitic agents survive by finding a suitable host within a certain time. They
have been able to improve their chances of finding a new host or surviving in the
environment by a number of different methods.
Reservoirs and parasite adaptability
A reservoir describes a suitable place for storing agents of infection. Once an
agent invades a host, there is normally a latent or waiting period while sufficient
organisms are produced before the main attack is mounted. If the host survives
but is not able to eliminate the invading organism, then the organism can
continue to live and reproduce in the host.
The relationship between the parasite and host is one of continual challenge,
what has been termed a biological arms race. When the parasite first attacks a
new species, the host attempts to eliminate it, resulting in a severe reaction. In
time, adaptation can occur, so that the reaction of the host diminishes and the
adaptability of the parasite increases. The parasite is then able to live in the host
with few ill effects forming an established population, and continuing with
minimal reaction from the host. The host then acts as a reservoir from which
parasites attack new hosts of the same species or attempt to colonize different
species.
Reservoirs can be humans, animals, vectors or the inanimate environment (e.g.
soil, water). Intermediaries may be required before the final reservoir host is
colonized. However, it is always within the parasite’s interest to improve its
reproductive capability, so if a new mutation arises which is beneficial to this
end, then this will be selected, generally to the host’s disadvantage.
The adaptability of parasites to their human hosts might even have advantages
for us. It is a strange irony that it may be preferable to actually introduce (rather
than control) these parasites to combat some allergic reactions.
Persistence
Another mechanism used by parasites to survive is the development of special
stages that resist destruction in an adverse environment. Bacteria can persist in
the environment by the development of spores, as with anthrax and tetanus
bacilli. Destruction of persistent organisms requires the use of antiseptics or
sufficient heat for a prolonged period.
Latency
The production by the organism of a developmental stage in the environment
that is not infective to a new host is called latency. This allows the parasite to
wait until suitable conditions develop before changing into the infective form.
1.2.3 The effect of the agent
If enough agents survive to infect a new host, they will produce illness, the
severity of which is determined by their toxicity and virulence. Infectious agents
produce a toxic reaction due to the foreign proteins they consist of or produce in
4
