Seasonality, a systematic periodic occurrence of events over the course of a year, is a well-known phenomenon in life and health sciences. Understanding seasonal fluctuations in diseases patterns presents us with a major challenge. To develop efficient strategies for disease prevention and control, we need to grasp the main determinants of temporal variations and their interactions. This paper will introduce the notion of seasonality by outlining several of its factors, using as illustrations respiratory and enteric water- or food-borne infections.