. A researcher studies the evolutionary relationships between influenza viruses by comparing the genomic sequences of the viruses found in pigs, ducks, and humans in a given year. When evaluating the viruses that had infected these hosts the previous year, she identifies a cross-species transfer from humans to pigs. This information supports the hypothesis that cross-species transfer leads to more severe disease if during the previous year _______ suffered a severe influenza pandemic.

Influenza is an infectious disease caused by influenza viruses, which can be divided into four types A, B, C and D. In humans, influenza A and B viruses are known to cause epidemics, while A viruses are known to cause pandemics. Influenza viruses are also found in birds and mammals (e.g., dogs, cats, cattle pigs and horses). Cross-species virus transmission refers to the transmission process of infectious viruses between hosts from different species. Several viruses that may cause severe diseases in humans have originated from animal species, e.g., avian influenza A virus that can be transmitted to humans through an intermediate host (e.g., pigs). Other examples of cross-species virus transmission include the Ebola virus and Hantaviruses. It has been suggested that cross-species transmission is a common phenomenon in rapidly evolving viruses.