where S is the number susceptible, I i is the number infected with strain i, and R the number of recovered or removed. We do not include births and deaths as we expect a zoonotic emergence, or extinction, to happen on a much shorter timescale than the human lifespan. We translate this model to a stochastic simulation of a multitype branching process, using the Gillespie algorithm [15] . The infection is seeded in a single random, susceptible host with the wildtype strain.