The US Equal Employment Opportunity Commission warns against employment discrimination that could arise because of the recent "Swine Flu" or H1N1 flu outbreak, called by some the "Mexican Flu."
It's not so clear that the Swine Flu would constitute a "disability" protected by federal law, but it is clear that the law prohibits employment discrimination on the basis of national origin, for example, discrimination against Mexicans. See Title VII statute, regulations, and guidance prohibiting national origin discrimination.
At the same time, the Americans With Disabilities Act (the ADA) regulates medical examinations and disability-related inquiries of employees and applicants, only permitting them if certain conditions are met.
For the EEOC's guidance about exams and inquiries of employees, see EEOC Enforcement Guidance on Disability-Related Inquiries and Medical Examinations of Employees Under the Americans with Disabilities Act, (defining medical examination and disability-related inquiry and explaining conditions under which they may be given to employees).
For the EEOC's guidance about exams and inquiries of applicants, see EEOC Enforcement Guidance on Preemployment Disability-Related Questions and Medical Examinations.