The World Health Organisation’s web site has good guidance and information about how to handle Swine Flu. So far it has spread to 29 countries.
Just as an employer has duty of care [Link] towards his employees he also has a right to refuse entry on the premises to those who are suffering from a disease like Swine Flu.
An employer also has the right to ask an employee to undergo medical check up if he suspects that the employee may be suffering from a disease which can be transmitted to other employees.
Here is the WHO link that is worth exploring.