In 2011, two laboratories published reports of mutational screens of avian influenza viruses, identifying variant which become transmissible through the air between ferrets. These viruses seem to overcome an obstacle which limits the global impact of natural H5N1.[10][11] In 2012, scientists further screened point mutations of the H5N1 virus genome to identify mutations which allowed airborne spread.[12][13] While the stated goal of this research was to improve surveillance and prepare for influenza viruses which are of particular risk in causing a pandemic,[14] there was significant concern that the laboratory strains themselves could escape.[15] Marc Lipsitch and Alison P. Galvani coauthored a paper in PLoS Medicine arguing that experiments in which scientists manipulate bird influenza viruses to make them transmissible in mammals deserve more intense scrutiny as to whether or not their risks outweigh their benefits.[16] Lipsitch also described influenza as the most frightening "potential pandemic pathogen".[17]