The following is a look at criteria the World Health Organization uses to categorize countries with SARS outbreaks.
Factors considered in issuing a WHO travel advisory, suggesting travelers postpone nonessential visits:
Magnitude of outbreak, determined by numbers of active cases and new cases emerging per day, averaged over three days.
If country has more than 60 active SARS cases and more than five new cases a day, it is placed in a category for consideration for a travel advisory.
Pattern of disease's spread. Countries where new daily cases occur in people already quarantined are in a better position than countries where the new cases are in people who have been in the community undetected for a few days, able to spread the disease.
Evidence that a location is exporting cases internationally.
A less important factor than the other two, but something WHO includes in the assessment.
In addition, WHO experts also examine the quality of SARS surveillance in a country, and any other relevant information, before making a broad assessment.
Beijing, Taiwan and four provinces in mainland China, Hebei, Inner Mongolia, Shanxi and Tianjin, are under a WHO travel advisory.
Factors considered in determining areas of local transmission:
Places where disease is spreading. Local transmission occurs when one or more people with SARS have most likely acquired infection locally. If no new locally acquired cases are identified 20 days after last local case died or was isolated, the area will be removed from this list.
Toronto, Hong Kong, Beijing, Taiwan and several provinces in mainland China are on the list.