What the law requires
Regulation (EC) No 852/2004, Annex II, Chapter VIII says that nobody who has or carries a disease that can be passed on through food, or who has infected wounds, skin infections, sores or diarrhoea, may handle food if there is a risk of contamination. The same chapter requires staff with such an illness or symptoms to report them to the business at once. In the UK, the Food Standards Agency pack Safer food, better business covers the same topic as fitness for work. Whether a health certificate is needed, and how often it is renewed, comes from your own HACCP plan, the national guide and the rules in your country.
Nobody who has or carries a disease that can be passed on through food may handle food (Regulation (EC) No 852/2004, Annex II, Chapter VIII). Record the health check or instruction that the rules in your country require.