Final answer:
Certain strains of Corynebacterium diphtheriae cause diphtheria because they produce a toxin, encoded by a bacteriophage, that leads to the severe symptoms of the disease.
Step-by-step explanation:
When we say that "only certain strains of Corynebacterium diphtheriae cause disease," we refer to the fact that within the bacterial species Corynebacterium diphtheriae, not all genetic variations or strains are capable of causing diphtheria.
The pathogenicity of particular strains is due to the presence of a temperate bacteriophage that carries the gene encoding the diphtheria toxin.
This toxin is responsible for the severe symptoms of diphtheria, such as the formation of a pseudomembrane in the throat that can lead to difficulty breathing and may require intubation. Diagnosis of the disease includes identifying bacterial cultures from throat swabs and detection of the diphtheria tox gene.