Anserke

An anserke is a kind of very spiny cactus that grows in hot deserts. The main element of the preparation that is drawn from it and which strengthens immunity against very many types of disease are its roots which sink more than six feet under the ground.
Drinking the anserke decoction gives advantage on Constitution checks against all nonmagical diseases for a number of days equal to the creature's Constitution modifier (minimum 1).