Several studies  have assumed that the index patients’ vomiting episodes provided sufficient evidence of airborne transmission. Nevertheless, as Booth suggested experimentally , vomiting may contaminate a wide range of surfaces that are often difficult to clean sufficiently, thus increasing the likelihood of further transmission of surface contamination. Moreover, Marks et al.  supported the possibility of airborne transmission because the time from exposure to illness decreased as the number of viral sources increased. However, as demonstrated by Lei et al. through simulations , an increase in the number of index patients could accelerate the transmission of surface contamination, thus increasing the accumulation of exposures and causing more rapid infection.