The seven days every millennium or so when Jirachi is awake are among the most dangerous in human history. We are all lucky that Team Magma and Aqua wasted their efforts resurrecting Kyogre and Groudon; had they thought to awaken Jirachi, whoever reached it first would have had their world-wrecking wish granted in an instant and could not have been defeated in battle.
Nearly one hundred years ago, a man conscripted into a brutal, global war declared a plague upon the house of those who had wronged him, and Jirachi interpreted his wish to mean the human race and brought forth would be the deadliest pandemic in human history. About fifteen hundred years ago, a well-meaning young woman wished for the Hoenn Empire, ruled by a cruel tyrant, to be stricken from the earth – and a meteor soon destroyed the imperial palace as a barbarian horde sacked the capital. But although the woman had hoped for a better tomorrow, she was rewarded only with centuries of anarchy and the collapse of a great civilization. Around one hundred and fourteen centuries ago, a scared and freezing Swablu wished for a warm bed at night. The ice sheets which covered our planet receded, and Man expanded around the world.
There is no telling when Jirachi will next wake; it is believed to do so only once every millennium, but human history is not long enough to confirm this pattern, and it is not in an even thousand-year cycle anyway. Jirachi, like the asteroid on which it fell to Earth 65 million years ago, is a disaster which can not be prepared for or mitigated, only averted by luck. So all we can do is hope the next person to find it will wish for it to grant wishes no more.