University of California, Irvine economist Ami Glazer said there’s no textbook to refer to on the issue.  “The previous recessions and depressions were caused by some collapse of the economic system or the financial system, — too many subprime loans and the like. This one is different,” Glazer said. “It’s because of an external virus. And so we know much less about what will happen than about previous recessions and depressions.”