Dear Michael,

I don't think there was a specific time when Christians "officially" switched to individual hosts, but I'm going to assume that most local churches started using them shortly after the invention of the bread iron, which would have been made commonly available centuries ago.  As for the reason why the Church didn't switch to individual communion cups for the precious blood, well that most likely is because of practical reasons.  It would be simpler and "safer" to use and purify only one vessel rather then several or many during the times in history when all laypeople would also receive.  However when breaking apart large loaves for several people there is a much greater risk of crumbs falling and getting out of control, so my guess is that as soon as there was a practical method for producing individual hosts the switch was made.

Hope this helps at least a little, but Church History isn't really my specialty.  I checked with the Catholic Encyclopedia (it's online) since it's been a great help to me in the past but I didn't see any specific dates or reasons.  A quick google search brought up a website that shows that there is art that dates back as early as the 6th century that appears to show individual wafers, but I can't personally guarantee if that's accurate or not.

Mr. Slavek