This excerpt from The Straits Times Interactive article Scientists' worldwide race against virus is particularly scary:

The latest theory is that 'silent spreaders' who carry the disease but show few signs may be most responsible for spreading Sars. 'There is some evidence of that,' Mr Iain Simpson, spokesman for WHO's communicable disease programme, said. 'It's something we're exploring but we don't fully understand.'

That might mean there is a Sars Sally out there, akin to America's Mary Mallon, who spread typhoid fever in the US in 1907 without showing any symptoms, leading scientists to dub her Typhoid Mary.