The infectivity of a patient with diphtheria is -
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
Till cough subsides
Till patient is febrile
Life long
For 15 days after infection
Communicable diseases