prophylactic

Definitions:

In med., defending from disease; preventive.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (adjective) (adj.) (adjective)
A medicine which preserves against disease.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
A preventive medicine.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
Preventing disease.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (adjective) (adj.) (adjective)
Warding off, or preserving from, disease.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adjective) (adj.) (adjective)
A medicine that prevents disease.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)

Usage examples:

But in the old days of idolatry, the hygienic and prophylactic practices alluded to by Herodotus, the greater cleanliness of the people, and the attention paid to the canals and drainage, probably prevented this malarious disease becoming the scourge which it is now.
- Personal Narrative of a Pilgrimage to Al-Madinah and Meccah by Sir Richard Francis Burton
Some few striking instances of immunity appear to offer a different basis for argument; but these might be due to the social position of the individual rather than to any constitutional temper: wealth and comfort, it must be remembered, have no small prophylactic value in such times.
- Two Years in the French West Indies by Lafcadio Hearn
But my uncle led me to his drugstore, where the stage was to call for me, and made me taste a little camphor; with this prophylactic Cervantes and I somehow got home together alive.
- Literature and Life by William Dean Howells