Viruses are acellular agents of disease.  All viruses contain nucleic acid, either DNA or RNA (never both), surrounded by a protein coat called a capsid.  The combination of nucleic acid and capsid is called the nucleocapsid.  The nucleocapsid can crystallize when dried, but the virus can still be capable of infecting and causing disease.

All viruses are obligate intracellular parasites that must successfully invade and pre-empt the normal metabolic activity of a host cell in order to replicate.  Most carry only nucleic acids, but a few, most notably the retroviruses and some hepatitis viruses, do carry their own specialized enzymes.

Some virions (viruses outside a host cell) are composed only of the nucleocapsid.  These are called naked viruses.  Others have a layer of plasma membrane taken from the previous host cell that surrounds the nucleocapsid.  This layer of membrane is called the viral envelope.

On the surface of either the capsid of naked viruses or the viral envelope are special proteins and glycoproteins called spikes.  The spikes aid in attachment of the virus to a host cell membrane.