Many, if not all, of the scents that have a tendency to reduce susceptibility to illness do so, at least in part, by reducing stress. At least some people who use the scent of lavender to induce relaxation and reduce stress are no more susceptible to illness than average. Some people who regularly inhale the scent of lavender would otherwise be under enough stress to impair their immune systems. Reduced stress diminishes susceptibility to illness only for people who are under enough stress to impair their immune systems to at least some degree.
Researcher: Research has shown that inhaling the scent of lavender has measurable physiological effects tending to reduce stress. It is known that intense stress can impair the immune system, making one more susceptible to illness. Ttherefore, it is likely that the incidence of illness among those who regularly inhale the scent of lavender is reduced by this practice.
Which one of the following is an assumption that the researcher's argument requires?