It has been adequately explained how subprime mortgages can become toxic assets. The question remains of "How did the losses incurred by subprime mortgage defaults spread throughout the economy?" The answer lies within mortgage-backed securities. It has long been practice to lump several mortgages together (all fixed income assets) as a security. The problem has long been pricing them properly. The factors influencing default are myriad making for a very complicated modeling problem. For some time, then, the only mortgage-backed securities traded were ones deemed riskless by association with the federal government via Fannie Mae or Freddie Mac. The breakthrough that allowed the massive amount of later trading came in 2000 with David X. Li's development of the Gaussian Copula[8], which allowed mortgage backed securities to be priced (along with countless other financial instruments) by using Credit Default Swap prices to model correlation of variables. The models utilizing the Gaussian Copula banked on the CDS market prices reflecting risk accurately. Credit Default Swaps, incidentally, are a kind of "bet" that some other asset will default. This may invite comparison to insurance, but neither party in the CDS has to have any interest in the subject of the bet: akin to buying fire insurance on your neighbor's house, with all the moral hazard it entails. This innovation allowed the creation of securities pooling diverse elements together with a very simple expression of the risk involved.