Well, in most cases, it's a thing we made up, but there is some portion of it that is just a measure of how much stuff we all make, and in this case, the economy is shit because people can't work and the people who are being forced to work are getting sick and not being able to work. Under any kind of economic approach, this virus would significantly reduce the amount of shit we're making, but only capitalism turns the "we're making less shit" problem into a "and now everyone is fucking homeless and sick"