About 6–9 or 1 mutation is needed for increased virulence in young and aged animals, respectively. This pattern, according to Baric, strongly resembles the progress of SARS and MERS in humans, suggesting that host age not only influences pathogenesis, but could also enhance animal-to-human cross-species transmission. It is possible that sufficient human-to-human and animal-to-human transmission events have occurred in the Middle East to model the role of aging in MERS-CoV transmission.