Contagious disease and Diphtheria

Contagious disease noun - Any disease easily transmitted by contact.

Diphtheria and contagious disease are semantically related. In some cases you can replace term "Diphtheria" with "Contagious disease", this terms are similar.

Diphtheria noun - Acute contagious infection caused by the bacterium Corynebacterium diphtheriae; marked by the formation of a false membrane in the throat and other air passages causing difficulty in breathing.

Contagious disease and diphtheria are semantically related. You can use "Contagious disease" instead a noun "Diphtheria".