enable or disable an apache virtual host?

To enable an apache virtual host run the following command:

sudo a2ensite example.com

To disable an apache virtual host run the following command:

sudo a2dissite example.com

Both of the examples above assume you're interested in enabling/disabling the site configured via the following file:

/etc/apache2/sites-available/example.com

Lastly, if you're interested in learning more about the a2ensite/a2dissite command, checkout the following notes:

#This manual page documents briefly the a2ensite and a2dissite commands.

#a2ensite  is a script that enables the specified site (which contains a
#<VirtualHost> block) within the apache2 configuration.  It does this by
#creating   symlinks   within   /etc/apache2/sites-enabled.    Likewise,
#a2dissite disables a site by removing those symlinks.   It  is  not  an
#error  to  enable  a  site  which is already enabled, or to disable one
#which is already disabled.

#The default site is handled specially: The resulting  symlink  will  be
#called 000-default in order to be loaded first.