As countries battle the COVID-19 pandemic, it is important to understand why certain public health policies and practices are adopted. This article seeks to explain basic communicable disease epidemiological concepts such as Reproductive Ratio, Serial Interval and Incubation Period. The article also demonstrates the effects of exponential growth on cluster size. These concepts help us to better understand the "disease dynamics" of COVID-19 and so enable us to understand better the policies and practices employed to combat COVID-19, e.g. social distancing, personal hygiene and mask-wearing.