Health literacy should be developed from early ages, but students, including preservice teachers, hold misconceptions about micro-organisms. The objective of this study was to analyse the knowledge that a sample of 102 Early Childhood Preservice Teachers (PST) had about micro-organisms and about cystitis. The PSTs performed a series of activities that included an inquiry-based laboratory activity. Their knowledge about micro-organisms was assessed by pre- and post-questionnaires, and about cystitis, by analysing the responses to an open-ended pretest, the inquiry reports and the performance of the PSTs when changing nappies on dolls. Results show a better understanding about the micro-organisms at the end of the study. In the case of cystitis, in the pretest PSTs did not identify bacteria as agents, but they did after the inquiry activity, and they also mentioned the proliferation of bacteria. Nevertheless, only 32% of PSTs changed nappies coherently with the knowledge.