This was based on the fact that trichinosis is prevalent in domestic pigs and in days of old there was no treatment for trichinosis. Hence the taboo.
Trichinosis (trik-ih-NO-sis), sometimes called trichinellosis (trik-ih-nuh-LOW-sis), is a type of roundworm infection. These roundworm parasites (trichinella) use a host body to live and reproduce. These parasites infect animals such as bears, cougars, walruses, foxes, wild boars and domestic pigs. You get the infection by eating the immature form of the roundworm (larvae) in raw or undercooked meat.
When humans eat raw or undercooked meat containing trichinella larvae, the larvae grow into adult worms in the small intestine. This takes several weeks. The adult worms produce larvae that travel through the bloodstream to different parts of the body. They then bury themselves in muscle tissue. Trichinosis is most widespread in rural areas throughout the world.