Describe the demographic, epidemiological and fertility transitions. Briefly define each and explain what caused the process to get started. What happens to the mortality and birth rates during each of four stages of the demographic transition? In general, where in the world has the demographic transition been completed and where not completed? For countries that have passed through the transitions to stage 4, what were the consequences from a public health perspective in terms of the kind of disease that became prevalent and the demographic composition and total size of the population? what is one of the possible threats to the future of the demographic transition?