A parasite swims faster in blood than in water.

Blood is packed with red blood cells, which creates a crowded obstacle course. Scientists wanted to know how Trypanosoma bruceiTrypanosoma bruceiA parasitic protozoan that causes African sleeping sickness in humans., a tiny parasite, manages to swim through this dense fluid. Because watching the parasite directly inside blood is very hard, researchers combined computer simulations with lab experiments. They tested how the parasite moves through blood cells and fluid filled with tiny spherical particles. They discovered that swimming in crowds actually helps the parasite. In computer models, its swimming speed increases by up to 50% at red blood cell concentrations matching real blood. It works because the surrounding particles change the friction against the parasite's beating flagellumflagellumA long, whip-like structure that the parasite uses to swim., which is its whip-like tail. The particles raise perpendicular friction more than parallel friction, giving each tail beat extra push. Lab tests confirmed this boost happens as particle concentration rises, especially when particles are similar to or smaller than the tail's wave.

Why it matters

Trypanosomes cause sleeping sickness, a fatal disease. Learning they swim faster in blood reveals how they spread through the body. It also uncovers a general principle: crowded particle suspensions can boost flagellar propulsion, which may apply to other microorganisms in dense fluids.

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