In the case of A.I.G., the virus exploded from a freewheeling little 377-person unit in London, and flourished in a climate of opulent pay, lax oversight and blind faith in financial risk models. It nearly decimated one of the world’s most admired companies, a seemingly sturdy insurer with a trillion-dollar balance sheet, 116,000 employees and operations in 130 countries. The insurance giant’s London unit was known as A.I.G. Financial Products, or A.I.G.F.P. It was run with almost complete autonomy, and with an iron hand, by Joseph J. Cassano, according to current and former A.I.G. employees.