The infectivity of a patient with diphtheria is -
Till cough subsides
Till patient is febrile
Life long
For 15 days after infection
Unless treated, the period of infectivity may vary from 14 to 28 days from the onset of the disease, but carriers may remain infective for much longer periods. Ref:PARK&;S TEXTBOOK OF PREVENTIVE AND SOCIAL MEDICINE , 23rd edition , Page 160
Communicable diseases