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.
Some people who regularly inhale the scent of lavender would otherwise be under enough stress to impair their immune systems is an assumption that the researcher's argument requires.