More on the concept of 'Cherem'...
http://www.torahweb.org/torah/2011/parsha/rsch_tazria.htmlWe have a similar idea regarding one put in "cherem". A cherem is a curse given by a beis din to a sinner that he should die. The Talmud points out (Moed Kattan 17a) that the three letters of the word "cherem" correspond to the ramach eivorim (248 parts) in the male body, implying that the cheren/curse is intended to effect all the 248 parts of the sinner's body, and thereby cause his death. One who dies while in cherem is considered like one who was put to death by the beis din, and his family may not observe aveilus (Shulchan Aruch Yoreh Deah chap. 345:4-5). Because the sinner (who was put into cherem) was the cause of his becoming like a "dead person", we therefore require him to observe aveilus over his own "death".
One who sinned and was sentenced to death by a beis din of twenty three dayanim is considered (in a certain sense) as if he were already a "dead man" even before his execution[1]. Similarly, when the beis din declares that one is in cherem, and when the Kohein declares the metzora to be "tomei", they too have the special status of being partially considered a "gavra ketilla".
The chumash tells us (Breishis 7:4) that when G-d finally decided to destroy all of mankind with the mabul, he waited seven days and only then caused the mabul to begin. According to our mesorah, Hashme's waiting seven days represents the fact that He observed a mourning period (shiva) for mankind. The commentaries point out that it is very strange to observe aveilus before the person dies just because he is about to die. The generally accepted understanding of this is that once G-d has declared (as a beis din) a death sentence on an individual (or on mankind), that person is now already considered partially a "gavra ketilla", and there is already room to begin observing aveilus just as there is in the case of one put into cherem.