 How does an animal virus jump species? A virus is a protein shell wrapped around a core of genetic information. All living things on Earth can be infected by viruses. Viruses are everywhere, but they are specific. Different viruses infect different hosts. Viruses can only reproduce inside living cells. To get inside, they must enter through structures called cell receptors, which act as gateways. Viruses have structures called spike proteins on their surfaces. Spike proteins act like keys which unlock cell receptors. If the spike protein fits, the virus can infect the cell. Once inside, the virus instructs the cell to make copies of the virus. But these copies are not perfect. The new viruses contain slight differences. Mutations. These mutations may alter the shape of the spike proteins, allowing the mutated virus to unlock different cell receptors. Even the cell receptors of other animal species. And humans. An animal disease that becomes infectious to humans is called a zoonosis. Any factors that bring people close to animals make zoonoses more likely to occur. About 60% of human infections have an animal origin. 3 out of 4 emerging infectious diseases are zoonoses.