Virulence
The
term
virulence
is
often
used
to
express
the
degree
of
pathogenicity
of
a
parasite.
The
term
is
derived
from
Latin
virulentus,
meaning
full
of
poison.
An
organism
such
as
the
typhoid
bacillus
that
invariably
causes
disease
is
said
to
be
highly
virulent,
whereas
Candida
albicans
which
causes
sometimes
disease
is
called
moderately
virulent.
Certain
organisms,
described
as
a
virulent
are
not
generally
regarded
as
disease
agents
as
for
examples
the
lactobacilli
and
strepto
cocci
found
in
curd
etc.
However,
it
should
be
noted
that
any
microbe
has
the
ability
to
change
genetically
and
become
virulent.
E.
coli,
for
example
was
long
considered
an
a
virulent
commensal
of
humans,
but
certain
toxin-producing
strains
have
been
isolated
that
cause
diarrhea
and
urinary
tract
diseases
in
humans.
The
virulence
depends
in
large
part
on
two
properties
of
a
microbe:
invasiveness
and
toxigenicity,
important
in
human
diseases
