Persons most commonly affected: Children of both sexes aged under 10 years but can occur in other age groups.

Organ or part of body involved: Respiratory tract, lungs.

Symptoms and indications: The mucous membranes lining the air passages are affected and after a one, or two-week incubation period, fever, catarrh and a cough develop. The cough then becomes paroxysmal, with continual bouts lasting up to one minute. At the end of each bout the child draws in the breath with a characteristic whooping sound. After about 2 weeks the symptoms start to lessen, although the cough may persist for some time.

Causes and risk factors: Infection comes from airborne bacteria, Bordetella pertussis, after close contact with a sufferer. The bacteria invades the nasal passages, respiratory tract and lungs, which produce a thick, sticky mucus in an effort to combat the invasion.