Final answer:
Epidemiology is the science that studies when diseases occur and their modes of transmission, it is closely related to public health, and plays a crucial role in developing health policies and managing disease outbreaks.
Step-by-step explanation:
The science that deals with when disease occur and how they are transmitted is called epidemiology. Epidemiology is the study of the occurrence, distribution, and determinants of health and disease in a population. As such, it is closely related to public health. Epidemiologists are professionals who collect data about specific diseases and track their spread to identify modes of transmission and other important statistical data related to a disease's impact on a given population. Their research includes the study of acute diseases, like the flu, and more chronic conditions, such as heart disease.
Epidemiology also involves the understanding of disease patterns, such as epidemics, which are sudden outbreaks, and pandemics, which are epidemics that spread through multiple populations or worldwide. The study of epidemiology does not limit itself to infectious diseases; it also includes non-communicable diseases, environmental factors, and even accidents.
The work of famous epidemiologists like John Snow has been foundational in the development of modern public health policies, which have greatly reduced the prevalence of many diseases throughout developed nations. However, epidemiology continues to be a vital field as new diseases emerge and known diseases evolve.