Information provision is only an effective behaviour-change strategy if the information is credible. A novel programme augments conventional hygiene instruction by showing participants everyday microbes under a microscope. Through a randomised evaluation in Pakistan, we show that this programme leads to meaningful hygiene and health improvements, while instruction alone does not. Traditional medicine, which offers an alternative disease model, may undermine learning by strengthening prior beliefs about hygiene. We show that believers in traditional medicine have smaller impacts, suggesting that traditional and modern medical beliefs are substitutes and that traditional medicine may exacerbate the infectious disease burden in this context.