A more thorough explanation:

A bill of health is a document that certifies the healthy condition of a ship's cargo and crew. It is issued by the port authority from which a vessel sails and is shown to the port authority at the ship's destination as proof that the ship's cargo and crew are disease-free.

A "clean" bill of health states that no contagious or infectious diseases were present at the port. On the other hand, a "touched" or "foul" bill of health states that the named disease was suspected, anticipated, or actually present.

For example, if a ship is leaving a port and the port authority determines that there are no contagious or infectious diseases present, they will issue a clean bill of health. This document will then be shown to the port authority at the ship's destination to prove that the cargo and crew are disease-free.

On the other hand, if a ship is leaving a port and there is a suspected or anticipated disease present, the port authority will issue a touched or foul bill of health. This document will also be shown to the port authority at the ship's destination to inform them of the potential risk of disease.