Hashem speaks to Moshe and instructs him to tell the Children of Israel: אִישׁ אוֹ אִשָּׁה, “…a man or a woman who commits any of man’s sins…”, וְהִתְוַדּוּ אֶת חַטָּאתָם, “They shall confess the sin they committed…” (Num. 5, 6-7) The command first refers to a man or a woman who commits a sin; in the singular. Then it switches to the plural, “they shall confess the sin they committed”. Why the change?
One answer given for the change from singular to plural is that when a person transgresses the entire community is guilty. The environment should be such that a person would feel embarrassed and guilt-ridden for perpetrating such an act. Since he did go astray, then the community at large carries part of the guilt for his action.
Another reason given is because: כל ישראל ערבים זה בזה, “All Israel are responsible one for another”. (Shav. 39a) That means that every Jew is morally responsible and accountable for his fellow Jew. If any Jew transgresses, all Jews should feel at fault.
The Jewish people are as one body or one entity. Just as when one limb hurts, the entire body feels the pain, thus too, if one Jew, any place in the world, is in danger then every Jew should feel obligated to come to his help.