Novel pathogens, viruses in particular, continue to spill-over from animal host reservoirs into human. Often, humans are dead-end hosts and the pathogens cannot be passed between people. For some viruses, which show some limited ability for human-to-human transmission, there is the threat that genetic modifications could render strains capable of effective transmission between people, and the potential for pandemic strains to emerge. Ongoing work is investigating emerging scenarios for different viruses, including avian influenza A(H7N9) and MERS-CoV, to better understand what an emergence event would look like from a public health perspective.