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.

INTRODUCTION: We conducted a descriptive study to evaluate any changes in practice behaviour regarding the provision of hydrocortisone and lignocaine (H&L) injections among doctors and how an H&L injection is priced following a disciplinary proceeding decision. A doctor had been fined SGD 100,000 for failing to obtain informed consent before an H&L injection. METHODS: We performed a survey shortly after the disciplinary decision to ascertain: (a) the category of the respondent; (b) whether the respondent provided H&L injections and how much he charged before the decision; and (c) after the decision. All members of the Singapore Medical Association and College of Family Physicians Singapore are doctors and were invited to participate. RESULTS: 1,927 doctors responded to the survey. Prior to the decision, 804 doctors did not perform H&L injections; this increased by 20.4% to 968 after the decision. The number of doctors who gave H&L injections decreased by 164 (14.6%), from the previous 1,123. Pre-decision, doctors who determined their own price for H&L injections charged a median pricing ≤ SGD 100. Post-decision, the median charge rose to > SGD 100 to SGD 200. At higher price bands, the number of doctors who charged > SGD 1,000 increased eight-fold, from eight to 65. CONCLUSION The study demonstrated how a disciplinary decision can affect practice behaviour, and specifically how doctors may choose to not offer a service, an example of defensive medicine through avoidance behaviour. It also showed how prices for a service can rise following such a decision, which demonstrates the concept of negative general deterrence in sentencing.