As a result of changes in the pathogen or in the host population, completely new infectious diseases may arise, or we may experience the reappearance of diseases previously considered to be under control. These are known as, respectively, emerging and re-emerging infections. Changed patterns of human population movement are often responsible for the development of such infections, with the spread of smallpox to the New World by European colonisers being a famous example. Frequently emerging virus infections are zoonotic in origin, that is, they are transferred to humans from animal reservoirs. HIV, for example, is thought to have developed from a similar virus found in monkeys.
While this book was in preparation, there was a sharp reminder of the ever-present threat of emerging viral diseases, in the form of a new viral disease called severe acute respiratory syndrome (SARS). The outbreak of this disease began in Guangdong province in southern China in November 2002. The Chinese authorities were heavily criticised for not reporting the extent of the outbreak until some 3 months later, by which time cases were appearing in many parts of the world, illustrating the role of increased intercontinental travel in the spread of such a disease. At its peak in April 2003, over 1000 new cases of SARS were being reported per week. The cause of SARS was quickly identified as a member of the Coronaviridae (single-stranded RNA viruses). Transmitted
A zoonosis is a disease normally found in animals, but transmissible to humans under certain circumstances.
by droplets from coughs and sneezes, it produces flu-like symptoms, but has a mortality rate of around 4 percent. Strict public health measures were brought into force, including restrictions in flights to and from affected areas, and the number of reported cases began to subside. In July 2003, the World Health Organisation announced that the final country, Taiwan, had been removed from its list of SARS-infected countries. By this time, SARS had claimed over 800 lives, mostly in China and Hong Kong, but with a number of deaths occurring as far afield as Canada and South Africa. Although apparently under control, isolated cases of SARS infections emerged in late 2003, and early 2004, most of which could be linked to laboratory workers. In January 2004 the Chinese authorities announced that a SARS vaccine was about to enter human clinical trials.
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