Illness and the Vital Sheath
There are historical examples of individuals who were caught in the middle of a major epidemic, such as the plague, who worked daily among the infected populace, yet failed to become ill. Invariably we can see that they maintained a strong vital envelope, or aura, around themselves and never accepted the suggestion of the contagious illness. There are also examples of those who maintained this strength for some time, but got worn down, and through stress and exhaustion, weakened their vital sheath sufficiently to eventually succumb to the disease.