Herd immunity is a form of indirect protection that applies only to contagious diseases. Wikipedia

Protection from infectious disease that occurs when a sufficient fraction of a population has become immune Wikidata

The indirect protection against the spread of a contagious disease in a given population caused by the immunity of a significant proportion of the population to that particular disease; the immunity may have been obtained through having survived the infection or through vaccination. Wiktionary

Form of indirect protection against a disease. Wiktionary (translation)