The green "wasabi" paste that accompanies the raw fish sushi (sashimi) is
thought to be an antibacterial substance.  I believe it is a horseradish
mixture.  When we lived in Japan we were told to be sure to put some of the
wasabi on our sushi.  Most of the sashimi is served that way.   Also, in
Japan folks are very particular about the fish being fresh.

The farther from a coast you live the farther the fish must travel.  It
wouldn't be out of order to ask a restaurant that serves sashimi how they
obtain their fish - how it is delivered, etc.  I should think a reputable
place would be glad to share the precautions they take to serve you the
freshest fish they can.  I live in New Hampshire, USA, and a local restaurant
has been quite picky about their fish sources, sending some back which they
found unacceptable.

Mardrey Swenson