This from a useful quick take on Freddie and Fannie by James Surowiecki:
The result of all this was that the companies reaped the rewards of the private sector while enjoying the security of the public sector. Seemingly insulated from all harm, they became reckless. They constructed a giant pyramid of debt on a very small base of capital (eighty-one billion dollars, by the most recent publicly available figures), and by May, 2008, either owned or guaranteed more than five trillion dollars in mortgages. As a result, even though just a small percentage of Fannie’s and Freddie’s mortgages are delinquent, the potential losses are huge. That’s why, in recent weeks, investors finally lost faith in them.
Even one percent of five trillion dollars is fifty billion dollars.