Why some viruses jump from animals to people and some don't

The Centers for Disease Control and Prevention estimates that three out of every four new infectious human diseases — including coronavirus diseases like COVID-19 — jumped from another species. Viruses are trying to infect new hosts all the time, but there are a lot of barriers that they need to get through before that can happen. First, a new virus needs to come in contact with humans, then overcome the body's natural defenses, and finally spread between people. But at any point along the wa...