If, during a meal, a person realizes that he touched an area of his body which is normally covered or that he scratched his head, he must rewash his hands. Even if he is not planning to eat any more bread, he must still rewash. If it occurs to him while he has a piece of bread in his mouth, he is forbidden to swallow until he rewashes. Above the elbow is considered an area which is typically covered. The poskim disagree about whether people who wear short sleeves that expose part of their upper arms must rewash after touching the exposed area. If a person touches actual tumah (e.g. a dead body) during a meal, he does not need to wash again.
ס"ב ס"ק ח' וט' ביה"ל ד"ה בשוק ביאורים ומוספים דרשו 15