Emotions do play a role as far as immune suppression from stress. Just like people prone to herpes outbreaks will get the outbreaks most often when they are stressed out, which weakens the immune system though adrenal dysfunction and elevated cortisol, which atrophies the thymus.
As for cancer though, this is not a modern phenomena. Cancer has been around for a long time. The first cancer virus was identified in 1910, and cancer was around long before that.