A pathogen is a bacterium that can cause disease or is virulent to a host. Most of these bacteria are found routinely in the environment and only cause disease under certain circumstances (opportunistic pathogens). What allows them to become infective? We study how non-pathogenic bacteria in the environment eventually evolve to be pathogens to humans and animals. We have determined that protozoa, which are single-celled organisms such as amoeba and ciliates, eat bacteria as their food. Bacteria have therefore, adapted certain traits that help them either escape being eaten or survive if consumed. These traits also lead to infection and disease in multicellular hosts such as humans. I will present some of the findings we have on how protozoa drive the development of virulence traits in opportunistic pathogens like Vibrio cholerae.