1. A method of assessing the presence of a pathogen in a sample comprising the steps of: a) irradiating the sample with first radiation having a spectrum centered at a first frequency and second radiation having a spectrum including a second frequency, wherein the first frequency exceeds the second frequency by a pre-determined non-zero frequency shift characteristic to the pathogen; b) detecting third radiation scattered from or transmitted through the sample at a third frequency that is different from the first and second frequencies, so as to form an image of at least a portion of the sample; and, c) analyzing the image to assess occurrence of one or more image artifacts each having one or more pre-determined features characteristic of the pathogen.