It can start off like an ordinary cold (catarrhal phase) for 7 to 14 days, but tends to worsen with periods of uncontrolled coughing. These areas sociated with a "whooping" noise during breathing in between bursts of coughing and often carry on until the child actually vomits. These episodes frequently cause the child to go blue and can be very worrying for the on looker. Between the periods or spasms of coughing the child seems fine. Often exercise or laughing brings on a spasm of coughing.
Attacks of whooping cough may last for six weeks or more and the reason that people worry is that there is a mortality rate, but this is low and happens mainly in children under 6 months. Sometimes whooping cough may be complicated by brain damage or fits or pneumonia. When adults get whooping cough the diagnosis may not be quite so clear cut and it may just seem like a prolonged, irritating, cough.
If you think your child might have whooping cough you should consult your doctor.

Causes

Whooping cough really refers to infections caused by the bacterium Bordetella pertussis, but an identical illness is produced by Bordetella parapertussis, Bordetella bronchiseptica and several types of adenovirus.
Bordetella pertussis has an incubation period of 12 to 15 days but this may be as long as 20 days. It is spread by droplets from the respiratory tract, rarely it may also spread on clothes, toys, etc.. The disease is most contagious during the cold-like initial phase (catarrhal phase).
 

Treatment

There is an antibiotic which is quite effective for whooping cough, but this is seldom used except in severe cases as the antibiotic has provedto be quite toxic in itself. There is one other antibiotic which seems to have some marginal effect, but essentially if your child gets whoopingcough you are in for a worrying few weeks and all you can do is be supportive and try symptom relief such as paracetamol and steam inhalations.
Complications can involve secondary infections eg pneumonia and collapse of the lung, and also effects on the nervous system eg encephalitisor fits (convulsions).