Maurice, what you can and cannot ask employees and others about their medical health depends very much on your region and the laws that apply.
The kind of medical / health surveying systems that may be expected for a food safety point of view are:
- New employee health review (pre employment or post employment)
- Visitor / contractor health survey prior to entry to factory
- Employee return to work health review following absence through illness
- Employee return to work health review following holiday to high risk regions
- Employees reporting illness at work