This is a 2-way crossover (method used to switch patients from one treatment arm to another in a clinical study), randomized (the study medication is assigned by chance), placebo-controlled (an inactive substance that is compared with a study medication to test whether the medication has a real effect in a clinical study) study in 3 cohorts (group of individuals with similar characteristics) ie, 18 healthy young female participants; 18 healthy elderly female participants; 18 female patients with a past history of MDD. Participants will be randomized to 1 of 6 possible treatment groups: Group 1: Treatment AB, Group 2: Treatment BA, Group 3: Treatment AC, Group 4: Treatment CA, Group 5: Treatment AD, Group 6: Treatment DA, where Treatment A is placebo vaccination, Treatment B is typhoid vaccination, Treatment C is psychosocial stress (Trier Social Stress Test [TSST]) followed by placebo vaccination, and Treatment D is psychosocial stress (TSST) followed by typhoid vaccination. Participants from each of the 3 cohorts will be randomized to these 6 treatment groups. In all the 6 groups the 1st treatment comes under Period 1 and 2nd treatment under Period 2 (eg, In Group 1: Treatment A [Period 1] and Treatment B [Period 2]) and there will be a minimally 7- and maximally 14- day washout period (period when no treatment is received) between the study periods. The study will consist of an eligibility screening examination (from 21 to 2 days prior to Day 1 of Period 1); a run-in visit (only prior to Period 1) in which eligible participants will be briefly explained about the cognitive test battery and immediately thereafter the baseline of cognitive function will be measured via these tests; 2 single-blind (a clinical study in which the person giving the treatment, but not the patient, knows which treatment the patient is receiving) treatment periods (2-way crossover); and a follow-up examination by phone (approximately 7 to 14 days after last treatment [Period 2]). For each participant, the maximal study duration will not exceed 8 weeks.