Host and environmental factors Poor socio-economic status(such as overcrowded damp housing and poor nutrition) arger family size ■ Maternal smoking Boys more than girls Prematurity-especially infants who have required artificial ventilation Congenital abnormalities of the heart or lungs Rarely, immune deficiency either congenital e.g. agammaglobulinaemia, or acquired, e. g. malignant disease or hiv infectionHost and environmental factors ◼ Poor socio-economic status (such as overcrowded, damp housing and poor nutrition) ◼ Larger family size ◼ Maternal smoking ◼ Boys more than girls ◼ Prematurity-especially infants who have required artificial ventilation ◼ Congenital abnormalities of the heart or lungs ◼ Rarely, immune deficiency, either congenital, e.g.agmmaglobulinaemia, or acquired, e.g. malignant disease or HIV infection