For as long as humans have lived in close proximity to animals, there has been the ever-present risk of infectious agents making the jump from them to us. This process is known as spillover: when a pathogen, like a virus, crosses a species barrier to infect a new host. It's not a rare phenomenon; some estimates have suggested that over 60 percent of infectious diseases that have emerged in humans have animal origins. A list of these infections – called zoonotic diseases – reads like a who's who of the most feared of human afflictions: Ebola, HIV, rabies, and bubonic plague, to name a few. Recently, reports of spillover cases of bird flu in humans, including the tragic death of a young girl in Cambodia, have sparked fears of more such events. But what is the science underlying spillover, and how worried should we be? How does spillover happen? There are lots of factors that determine whether a particular disease will spill over into the human population. To start with, you need