“The metaphor I use is that years ago, clinicians used to look at ‘fever’ as one disease,” said John McGrath, an epidemiologist with Australia’s Queensland Centre for Mental Health Research and one of the world’s authorities on quantifying populations of mentally ill people. “Then they split it into different types of fevers. And then they realized it’s just a nonspecific reaction to various illnesses. Psychosis is just what the brain does when it’s not working very well.”