<prompt> The infectivity of a patient with diphtheria is -Till cough subsidesTill patient is febrileLife longFor 15 days after infection  <response>: 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 