Gary Greenberg looks at why it has grown to include more and more afflictions:
The idea that disease is a biochemical entity originated only in the mid-nineteenth century, when scientists like Louis Pasteur began to spot pathogens under microscopes and chemists like Paul Ehrlich began to fashion drugs that could kill them. In the first century after those discoveries, the new idea wrought miracles, turning illnesses like strep throat and diabetes, which once routinely killed us, into nuisances.
But this idea has become a myth, a story that controls our understanding of the world. And the myth has spawned the tendency to try to turn all our suffering into the kind of diseases that can be identified and targeted in this fashion, in the hope that they will then go the way of smallpox and scarlet fever. In its thrall, we have come to expect from doctors what they cannot possibly give: a certainty, based in blood tests and tissue cultures, about everything that ails us and how to fix it. And our doctors have responded by trying to provide what we are asking for—in the case of psychiatry, a thousand-page-long catalog of psychological suffering cast in the rhetoric of scientific medicine.