Hajj would have been a perfect opportunity for Allah to give us some evidence for his existence. Every Muslim, if they can afford to and are well enough, should journey to Mecca for Hajj, a pilgrimage during the 8th to 12th day of Dhu al-Hijjah, the 12th and last month of the Islamic calendar. (Pilgrims can go at other times in the year, called Umrah, but it is Hajj that forms one of Islam's Five Pillars).
Millions of pilgrims attend, and, as one would expect with so many people in so small an area, the risk of infection rises massively, hence frequent calls to remind pilgrims that they must have the available meningitis vaccines for example.
Of course, this seems a little sadistic if Allah does exists. He has essentially brought a lot of people together and not bothered with a wee miracle to keep the dieseaes (he created) at bay (which, given the fact he made the universe and all, you'd think wouldn't be that hard).
People die on Hajj.
What a waste.