Well, no. If there had been obvious causes for major depression, they would have been found by now. There may be some infectious agents who induce depressions (

Well, no. If there had been obvious causes for major depression, they would have been found by now. There may be some infectious agents who induce depressions (like hepatitis), and some have not yet been discovered, but I highly doubt that a major infectious cause for major depression is still "hiding".