The concept of herd immunity applies to:

a. All infectious diseases

b. All contagious diseases

c. All infectious and all contagious diseases

d. Neither all infectious nor all contagious diseases

The correct answer is b. All contagious diseases.

Explanation:

A contagious disease can be transferred from one individual to another individual. Herd immunity is the immunity of a community which resist the spreading of all contagious disease in those people who are not immune to that disease.

Herd immunity only works when a large number of individuals in that community is immune to that particular contagious disease. When a large large number of individuals are immune then the contagious disease is not able to penetrate through these individuals and the small population of unvaccinated individuals remain unaffected and protected.

Thus, the correct answer is b. All contagious diseases.