Pigs used to be kept in very disgusting conditions, and were very likely to have parasites. So people would get sick from eating them, especially if it is not cooked enough. Modern farming now is much more careful about keeping them in a good condition with no diseases or parasites. But they still have a risk of some parasites, and it is important to cook it enough to kill anything that might be in there. Like with chicken, it has to be cooked all the way.
There's also a risk of getting very sick from things like clams and oysters, especially if they are not kept refrigerated at a very low tempurature which they did not have then. So some people got very sick or died from eating these.
This is the reason why they don't eat pork or shell fish. Because enough of them got very sick from eating them that they decided that it's not a good idea.