a, Artistic rendering (not to scale) of a typical transmission site where cercariae (immature, human infectious stage of the parasites) emerge into freshwater from snails and aggregate near the water surface8. b, Artistic rendering of the three distinct modes displayed by cercariae in their intermittent swimming behaviour (Supplementary Video 1) termed (i) tail-first mode, (ii) free-sinking mode and (iii) body-first mode. Arrows indicate the direction of motion. Scale bar, 100 μm. c, Computationally tracked snapshot of cercariae swimming in a vertical chamber (Supplementary Methods) showing tail-first swimming (red) and sinking (blue) trajectories for 10 min (Supplementary Movie 2). The resulting tracks of organisms have a strikingly columnar nature (Supplementary Movie 2) with very little drift in the horizontal direction. This is due to the fact that cercariae are bottom-heavy swimmers (Supplementary Fig. 2) and undergo a gravitational realignment such that their longitudinal axis is aligned with the vertical with the body pointing down. Scale bar, 1 mm.