Final answer:
In the 1900s, early on, infectious diseases were the leading cause of death, but as the century progressed, non-communicable or chronic diseases like heart disease and cancer became the top killers, especially in developed countries.
Step-by-step explanation:
The top killers in the 1900s were predominantly infectious diseases. This was especially true in the early part of the century, where diseases like tuberculosis, diarrhoea, dysentery, enteric fever, typhoid, and cholera were widespread due to urban conditions that facilitated their spread. These communicable diseases were particularly deadly until cultural reforms and advancements in public health in the latter part of the 19th century began to reduce their impact.
However, as we moved further into the 20th century, the leading causes of death shifted towards non-communicable diseases, such as heart disease, stroke, diabetes, and cancer, particularly in developed countries. These are largely considered chronic diseases, which tend to affect people over a longer period, and are more prevalent among older adults. Lifestyle choices, such as diet and exercise, along with genetics, play a significant role in the prevalence of these diseases.
Infectious diseases remained a crucial health concern in developing countries well into the late 1900s, where they continued to cause a large number of deaths annually, exacerbated by factors like malnutrition, poor sanitation, and inadequate medical care.