If a bee feels provoked or if people it is then alarmed and uses it self defence weapon. There is a rule it can only sting if the bee is Threatened.
The sting at the end of the bees body is really a fine tube it is barbed too. If a bee stings it could drop a little poison into the human body.
The bee only stings Once because if it gets knocked off then he would get badly injured then die. Sometimes they say it naturally pulls it on sting out and they might live too.
Sometimes, a bee can sting much as often as we allow it, if we do not knock them off the place they stung.