Annals of Ibadan Postgraduate Medicine. Vol. 14 No. 2, December 2016 56 Prior to the beginning of the 20th Century, infectious diseases accounted for high morbidity and mortality worldwide. The average life expectancy at birth was 47 years (46 and 48 years for men and women respectively) even in the industrialized world. Infectious diseases such as smallpox, cholera, diphtheria, pneumonia, typh...