Severe Acute Respiratory Syndrome (SARS) is considered the first pandemic of the 21st century. Within months after emerging in the Guangdong province in mainland China, it had affected more than 8000 patients, causing 774 deaths in 26 countries over five continents. This paper reviews the causes, epidemiology, and the clinical features of the disease.Causes In November of 2002, an anomalous form of pneumonia emerged in Foshan, Guangdong Province, mainland China. The disease spread across Eastern Asia, beginning in Hong Kong and then to Vietnam, Singapore, Canada, and so on (Fig. 1) in February and March 2003. Severe Acute Respiratory Syndrome is now known to be caused by a coronavirus, which are a family of enveloped viruses, meaning it has outer layer or coat made from the plasma membrane, which allows it to infect other cells and survive, with a positive single stranded RNA. They can cause diseases associated with respiratory, gastrointestinal, liver and neurologic tracts in humans and animals, but are most frequently linked with the common cold. Severe Acute Respiratory Syndrome Coronavirus, SARS-CoV, presence has been illustrated through the use of reverse transcriptase polymerase chain reaction (RT-PCR) and the isolation of of the virus from respiratory secretions, feces, urine, and lung biopsy tissue specimens. Reverse transcriptase is an enzyme used to generate complementary DNA from an RNA template, mainly associated with retroviruses, such as SARS. The technique of RT-PCR is commonly used in molecular biology to detect RNA expression by using the ability of RT to synthesize a complementary DNA from mRNA transcripts, then using PCR to amplify regions of interest. This indicates the infection is not unique to the respiratory tract, but affects other systems, like digestive and excretory. An experimental infection of SARS-CoV was conducted on Cynomolgus macaques, an important non-human primate in biomedical research, who Osterhaus, Fouchier, and Kuiken (2004) found to have “developed a disease comparable to SARS in humans”(p.1). The results of this experimental infection suggests the causation of SARS in humans can be traced back to the necessary presence of SARS-CoV, although it is not confirmed if the transmissibility or severity of the disease can be enhanced by microbes or other cofactors. All of these factors prove that SARS-CoV is a new group within the coronavirus family (Fig.1). Around six months after the disease was first recognized, the World Health Organization (WHO) coordinated an international investigation, with the causative agents of SARS being one of the main research priorities. Due to seroepidemiological data, or data involving the identification of antibodies to specific antigens in populations, indicating that the SARS-CoV was not a human originated endemic, it seemed probable that this virus had crossed the species barrier from animal to human. Studies were focused mainly on wild animals captured and marketed for culinary purposes due to the fact that the majority of the early cases had reportedly occurred in people handling these wild animals sold as exotic foods. A live animal retail market was investigated in Shenzhen, a major city in the Guangdong province, where animals, mainly wild, with one domestic, were sampled . These animals originated from different regions of southern China, only coming into contact with their arrival to the market, and had remained in the markets for irregular amounts of time. Nasal and fecal samples were taken, stored in medium 199, a solution used to encourage virus production, with bovine serum albumin, a protein concentration derived from cows, and antibiotics, as well as blood samples for serology. Anonymous samples from the traders were also taken, with informed consent, and animals were examined by veterinary surgeons beforehand to ensure they were free of apparent disease. These animal and human samples were used for comparison.