Viruses -
They
are
a
cellular,
infectious
agents.
Since
they
have
no
associated
metabolism
one
can
argue
whether
they
are
organisms
at
all.
They
have
no
such
activity
except
replication,
that
too
only
in
cytoplasm
of
live
cells.
They
consist
of
a
fragment
of
nucleic
acid
(DNA
or
RNA,
never
both)
surrounded
by
a
layer
of
protein.
They
cause
important
diseases.
Viroids are minute infectious agents of plants which are composed of single naked RNA molecules.
The first report of these pathogenic RNA molecules in the early 1970s aroused much interest, since it was previously thought that infectious nucleic acids could survive outside a cell only if they are encapsulated in a protein capsid.
