The PROTETEC study is a parallel, double-blind, placebo-controlled 4-weeks intervention with probiotics in healthy volunteers. In this study, the effect of probiotic intervention vs placebo on several infection markers in response to an ETEC challenge is investigated. Participants will be randomly assigned to the probiotic or placebo group (n=30 per group). Subjects will be instructed to maintain their usual pattern of physical activity and their habitual food intake, but to standardize their dietary calcium intake. After an adaptation period of 2 weeks, subjects will be orally infected with a live, but attenuated, ETEC vaccine (strain E1392-75-2A; collection NIZO food research; dose 10E10 CFU). Before and after infection, a diary will be kept to record all food and drinks consumption (2x2 days) to assess the habitual dietary intake, as well as for daily recording of bowel habits and frequency and severity of gastrointestinal complaints. The following biological samples will be collected: 4x10 ml venous blood, a single fecal bolus (for screening) and 7x24 hrs feces. Blood is sampled for immune response analyses and the fecal samples are collected to quantify several infection- and immune system markers, to determine probiotic excretion, and to verify dietary calcium intake.