  Motivated by recent experimental measurements, the passive diffusion of the
bacterium \textit{Leptospira interrogans} is investigated theoretically. By
approximating the cell shape as a straight helix and using the
slender-body-theory approximation of Stokesian hydrodynamics, the resistance
matrix of \textit{Leptospira} is first determined numerically. The passive
diffusion of the helical cell is then obtained computationally using a Langevin
formulation which is sampled in time in a manner consistent with the
experimental procedure. Our results are in excellent quantitative agreement
with the experimental results with no adjustable parameters.
