This
is defined as the
invasion of the body by an organism that has the potential to cause
disease, it occur when a parasite is growing and multiplying within a
host.
The nature
of an infection varies with respect to,
·
Severity
·
Number
of organisms involved
·
Location
of infection
Infection with an
organism does not necessarily mean disease, since the immune system in most
cases will be able to eliminate the invading organism before disease.
An
infectious disease
Any change from a state of health in which
parts or the enire body is not properly adjusted or capable of carrying out its
normal function due to the presences of parasite or its products.
Any parasitic organism
that produces disease is called PATHOGEN.
The ability of a
pathogen to cause disease is known as PATHOGENICITY.
Sometimes, an
infectious organism may enter a latent state in which there is no shedding of
the organism and no symptom present within the host. This latency may be either
intermittent or quiescent.
Intermittent
latency –
Is exemplified by herp virus that
cause cold sore (fever blister). After an initial infection, the symptom
subside; however, the virus remain in the nerve tissues and can be activated in
weeks or months later by factors such as stress, sunlight and change in immune
system.
Quiescent
latency – the organism persists but remain inactive for a
long period of time, usually years e.g. vericella
Zooster virus causes chicken pox in children and remain after the disease
has subside in adults, under certain conditions, the same virus may erupt into
a disease called shingles.
Host/parasite
relationship depends on
· Number
of organism present in the host.
· Virulence
of the organism.
· Host
defence or degree of resistance.
· Nature
of inoculums.
The greater the number
of parasites presents in a host, the greater the livelihood of the diseases. However,
a few organisms can cause disease if they are extremely virulent or if the host
resistance is weak. A host resistance can drop so low that its own microbiota
may cause disease, such a disease is called ENDOGENOUS
DISEASE because the organism usually come from the hosts own body e.g. Actinocycosis caused by actinomyces is really part of the normal
flora of the mouth.
Endogenous infections are
infections already present in the body.
Virulence
-
is the degree of pathogenicity and is determined by three characteristics,
·
Invasiveness
·
Infertility
·
Pathogenic
potential
Invasiveness
– This is the ability of a pathogenic organism to spread to other tissues.
Infectility
– This is the ability d the organism to establish a focal point of infection.
Pathogenic
potential – This is known as the degree to which the organism
causes morbid symptoms.
The function of the
immune system is self discriminating, and it is important in defending the body
against infectious organisms or eliminating modified or altered cells. This
takes place when.
·
The
number of infectious organism is high
·
When
the immune system is compromised
·
Virulence
of the organism is high
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