When will a small disease outbreak grow into an epidemic? Go back to the model of fire spreading from tree to tree. A high wind speed in the forest, blowing sparks quickly from tree to tree, is like a virus that is highly contagious. A high density of trees is like people living close together (in cities, for example). When the infectability and density cross a critical threshold, small outbreaks erupt into epidemics. When they fall below that threshold, small outbreaks die out quickly. That’s the epidemic phase transition.
