If a pool is full of light balls, a swimmer can push off them and move faster than in clear water. That's roughly how trypanosomes behave: these tiny parasites cause sleeping sickness. In human blood, red cells take up almost half the volume, and scientists used to think they got in the parasite's way. But it turns out that in such a thick mixture, trypanosomes actually move faster.

A trypanosome has a flagellum, a long, flexible extension that beats in waves and propels the cell forward. In pure fluid, the flagellum meets nearly the same resistance in every direction, so the push is weak. But when particles are floating around, they hinder sideways motion more than forward motion. The flagellum starts pushing off the particles like an oar against water, and the parasite accelerates.

This mechanism helps trypanosomes survive in blood and may work in other microorganisms, such as sperm in mucus. Understanding it could aid in designing devices for rapid diagnosis of parasitic infections.