Taboo

Food taboos exist in a variety of cultures and religions.  For me, pork has always been a biological taboo because of the parasites it carries that can be transmitted to humans.  However in controlled conditions pigs can be grown with relatively clean meat and if cooked thoroughly parasites can be killed.  But that's the case with lots of meat products in today's factory system - raw and undercooked can mean a brush with death.

So what is clean?  It depends on if you're talking biology or Leviticus.  It is clear that many of the dietary laws in Leviticus 11, make a lot of biological sense, particularly for the period. 

I have been asked a million times why I eat "shellfish" - the assumption being that they are scavengers and many correlate them to undesirable insects.  I'm no biologist but I do have a basic knowledge of scientific classification, and while mollusks are distantly related to snails and slugs, and crustaceans are distantly related to insects and arachnids - they are not the same.  In fact, they only have the phylum in common and follow a different trajectory down two ranks to order.  Not to mention that attempting to apply terrestrial rationale to marine animals can be tricky.

The biggest issue is that people overgeneralize and apply terms such as scavenger loosely without understanding the organism's role in an ecosystem and what they are scavenging.  I love the flesh of filter feeders such as oysters, mussels, clams, and scallops.  I also don't mind squid, which falls into a different class of mollusk.  My concern is their ecosystem because whether or not you have fins or scales, if you're swimming in a filthy habitat then you're certainly not clean.