The virus thought to cause Sars is constantly changing form, say scientists - which will make developing a vaccine difficult. The Beijing Genomics Institute reported that the virus is "expected to mutate very fast and very easily". Other experts have warned that, once established, it could be particularly hard to stop the Sars virus causing problems. Sars appears to be caused by a new strain of a coronavirus which may have "jumped" from animals to humans in the southern Chinese province of Guangdong. So far the strain has killed more than 200 people, mainly in China, Hong Kong, Canada and...