Viruses
are
by
far
the
most
____________________
biological
entities
on
Earth
and
they
outnumber
all
the
____________________
put
together
.
They
infect
all
types
of
cellular
life
including
animals
,
plants
,
bacteria
and
fungi
.
However
,
different
types
of
____________________
can
infect
only
a
limited
range
of
hosts
and
many
are
species
-
specific
.
Some
,
such
as
smallpox
virus
for
example
,
can
infect
only
one
species
in
this
____________________
humans
,
and
are
said
to
have
a
narrow
host
range
.
____________________
viruses
,
such
as
rabies
virus
,
can
infect
different
species
of
mammals
and
are
said
to
have
a
broad
range
.
The
____________________
that
infect
plants
are
harmless
to
animals
,
and
most
viruses
that
infect
other
animals
are
____________________
to
humans
.
The
host
____________________
of
some
bacteriophages
is
limited
to
a
single
strain
of
bacteria
and
they
can
be
used
to
trace
the
source
of
____________________
of
infections
by
a
method
called
phage
typing
.
