The intimacy of the relationship between hosts and their parasites is astonishing. Host and parasites have co-evolved for a long period of time. Due to their relevance for public health issues, infectious diseases, in addition, provide often extended spatio-temporal data sets. We use this valuable information to develop models and theory. Our aim is to understand the interplay among environmental factors, the dynamics of disease transmission and the inherent stochasticity of the infection process. We have worked on malaria in the past. Nowadays, we are using citizen-science data on the expansion of the Tiger Mosquito to develop models that help assess potential infection risks of the diseases transmitted by this mosquito.