Introduction

Severe acute respiratory syndrome (SARS) is a new infectious disease which was first recognized in late February 2003, when cases of an atypical pneumonia of unknown cause began appearing among staff at a hospital in Hanoi. Within two weeks, similar outbreaks occurred in various hospitals in Hong Kong, Singapore and Toronto.

On March 15, the World Health Organization (WHO) issued emergency travel recommendations to alert health authorities, physicians and the traveling public to what was perceived to be a worldwide threat to health. The travel recommendations marked a turning point in the early course of the SARS outbreak. Areas with cases detected before the recommendations were issued, namely Vietnam, Hong Kong, Singapore and Toronto, experienced the largest and most severe outbreaks, all characterized by chains of secondary transmission outside the healthcare setting. After the recommendations had been issued, all countries with imported cases, with the exception of provinces in China, were able, through prompt detection of cases and isolation of patients, either to prevent further transmission or to keep the number of additional cases very low (WHO. SARS: Status of the Outbreak).

After the disease had moved out of southern China, Hanoi, Hong Kong, Singapore, and Toronto became the initial "hot zones" of SARS, characterized by rapid increases in the number of cases, especially in healthcare workers and their close contacts. In these areas, SARS first took root in hospital settings, where staff, unaware that a new disease had surfaced, exposed themselves to the infectious agent without barrier protection. All of these initial outbreaks were subsequently characterized by chains of secondary transmission outside the healthcare environment (WHO. SARS: Status of the Outbreak).

Now, at the beginning of July, SARS appears to be under control. It might not be all over, though. Toronto, after having had no new cases for more than 20 days, experienced a second outbreak with cases