During the process that turns an animal virus into a strain that may infect humans, viruses that originate as harmless pathogens in humans are transferred to animals where they mutate to produce new strains that can be dangerous to humans. The viruses don't usually evolve into anything that poses much of a threat to humans. Viruses that originate in animal hosts like pigs or birds combine to exchange genetic material and then continue to mutate in their animal hosts. What is the main process that leads to the development of new strains of viruses that can infect humans?
1) Transfer of harmless human pathogens to animals
2) Evolution of harmless human pathogens into dangerous strains
3) Exchange of genetic material between animal hosts
4) Mutation of viruses in animal hosts