Thomas J. R. Hughes

An epidemic is like a forest fire: the flames race toward where there are more dry branches. Similarly, a virus spreads faster where people live in closer quarters. But old mathematical models described infection as an ink blot spreading evenly across paper.

Imagine a river that doesn't flow in an open channel but through the inside of a sponge, with its currents shifting from place to place. That's a good way to think about the brain's glymphatic system: a network of ultra-fine channels around blood vessels, through which cerebrospinal fluid seeps into the tissue and washes away the waste that has piled up during the day.