Methods for aiding in the diagnosis of disorders including, but not limited to, PDDs (Pervasive Development Disorders), Dysautonomic disorders, Parkinson's disease and SIDS (Sudden Infant Death Syndrome). In one aspect, a diagnosis method comprises analyzing a stool sample of an individual for the presence of a biological marker (or marker compound) comprising one or more pathogens, which provides an indication of whether the individual has, or can develop, a disorder including, but not limited to, a PDD, Dysautonomia, Parkinsons disease and SIDS. Preferably, the presence of one or more pathogens is determined using a stool immunoassay to determine the presence of antigens in a stool sample, wherein such antigens are associated with one or more pathogens including, but not limited to, Giardia, Cryptosporidium, E. histolytica, C. difficile, Adenovirus, Rotavirus or H. pylori.