Which people developed disease and why

This epidemiological transition is the result of a series of interrelated factors:  

• Demographic changes: the reduction in childhood mortality leads to a decrease in fertility rates. As a consequence, a higher percentage of the population reaches the adult age and develops adult-related diseases. 
• Changes in risk factors: this includes changes in the prevalence, distribution and/or virulence of pathogenic organisms, environmental changes – frequently linked to human activity- that can cause disease, and social and cultural factors such as lifestyle and diet. 
• Modern medicine practices: vaccines are without doubt the greatest public health success in history – every year they avoid 2 to 3 million deaths by diseases such as diphtheria, tetanus and whooping cough, they have allowed the eradication of smallpox and practically that of polio. The discovery of penicillin in the 20th century was another milestone in modern medicine and, ever since, antibiotics have saved hundreds of millions of lives.