We compared virus-specific antibody and T-cell responses to influenza virus vaccination in 32 caregivers of Alzheimer's disease (AD) patients and matched control subjects. Caregivers showed a poorer antibody response and virus-specific T-cell response following vaccination compared to the control subjects as measured by fourfold increases in antibody titers to the vaccine and lower levels of virus-induced IL-2 levels in vitro. We performed a second study in which forty-eight medical students were inoculated with a series of three injections of the hepatitis-B (HEP-B) vaccine to coincide with the third day of three, three-day examination blocks. Twelve of the 48 medical students seroconverted after the first injection; these students were characterized by falling into the lower stressed/lower anxiety group of students. Students who reported greater social support and lower anxiety and stress demonstrated a higher antibody response to the vaccine and a more vigorous T-cell response to HEP-B surface antigen at the end of the third examination experience. The differences in antibody and T-cell responses to HEP-B and influenza virus vaccinations provide a demonstration of how stress may be able to alter both the cellular and humoral immune responses to vaccines and novel pathogens in both younger and older adults.