The goal of this study is to learn if short-term changes in the immune system alter how we process information and experience fear. The main questions it aims to answer are: Do people who receive typhoid vaccine respond differently than those who receive a placebo saline vaccine? Do people who receive typhoid vaccine experience changes in how they think and feel? Participants will: Attend four appointments at the San Francisco VA Health Care System; Receive typhoid vaccine or placebo at one of the visits; Have their physiological responding measured while listening to sounds; Complete questionnaires and psychological tests.