Dmitry A. Fedosov

There's a leak in the garden hose. You patch the hole with duct tape, but the water presses harder and harder, and sooner or later the patch gives way. A growing clot in a blood vessel behaves much the same way. When the vessel wall is damaged, tiny blood platelets rush to the scene.

A swimmer in a pool filled with light balls pushes off them and swims faster than in clear water. That is roughly how a trypanosome, a single-celled parasite that causes sleeping sickness, behaves when it enters blood, where red cells occupy nearly half the volume.