Clinical observations suggest that gut and dietary factors transiently worsen, and in some cases appear to improve, symptoms in a subset of persons with autism spectrum disorders (ASDs). This lecture examines the possible role of infectious agents in the causes and symptoms of ASDs. It discusses the effects of a series of infective and dietary agents of increasing incidence that are capable of inducing brain changes and complex behaviours (i.e., aggression, biting, food craving, perseveration, social impairment, hyperactivity, obsessive compulsive activity, sensory sensitivity, seizure) in humans and experimental animals. Dr. MacFabe presents his current research examining the ability of a panel of gut bacterial metabolic products (i.e., short chain fatty acids) associated with antibiotic induced clostridial infections, and their ability to produce brain neuroinflammatory, metabolic, epigenetic and behavioural changes closely resembling those found in ADSs. It discusses the hypothesis that ASDs are produced by pre- or post-natal antibiotic resistant clostridial infections in sensitive subpopulations.