True regarding peusis is all, except -

<p> Peussis/Whooping cough:- An acute infectious disease ,usually of young children, caused by B.peussis. Clinically characterised by insidious onset with mild fever and an irritating cough, gradually becoming paroxysmal with the characteristic whoop often with cyanosis and vomiting. Also known as "Hundred day cough". AGENT FACTORS:- 1. Agent-B.peussis. Small percentage of case are caused by B.parapeussis, adenovirus, parainfluenza virus. 2. Source of infection- B.peussis infects only man. The source of infection is a case of peussis. Chronic carrier state doesn&;t exist. 3. Infective material- Nasopharyngeal and bronchial secretions. Objects freshly contaminated by such discharges are also infective. 4. Infectious period- most infectious during catarrhal stage. Infective period may be considered to an extend from a week after exposure to about 3 weeks after the onset of paroxysmal stage although communicability dimnishes after catarrhal stage. 5. Secondary attack rate-90% in unimmunized household contacts. HOST FACTORS:- 1. Age- primarily affects infants and pre-schoolchildren. Highest incidence found below 5 years of age. Infants below 6 months have highest moality. 2. Sex- more among female. 3. Immunity-Recovery from whooping cough/adequate immunization is followed by immunity. Infants are susceptible to infection from bih because maternal antibody doesn&;t appear to give protection. 4. No cross immunity with B.parapeussis. Mode of transmission:- Mainly by droplet infection and direct contact. Each time the patient coughs,sneezes or talks, the bacilli are sprayed into the air. The role of fomites in the spread of disease appears to be very small , unless they are freshly contaminated. Incubation period-7-14 days , but not more than 3 weeks. {Reference: park&;s textbook of preventive and social medicine, 23rd edition, pg no.167}</p>