what fatal dangers / problems did mankind face centuries ago, but are no longer a threat today?
Answers
Plagues and diseases
Centuries ago before vaccines were made, there where many diseases that killed hundreds and thousands of people like polio, tuber culosis, etc. But one of the main dangers were plagues. For example the Black Death which was the reason behind the death of millions of people. Also the bubonic plaque also led to the death of thousands of people. At that time they didn't have ventilators, vaccines,and other types of medical equipment and because of this the death toll was very high. For comparison if the black death plaque occured in the 21st century, it would kill no more than a few thousand people before the vaccine is made for it and if the corona pandemic would have occured 200 years before today, it would have killed at least 10-20 million people.
Famines
Centuries ago, famines where also one of the main causes of death. For example about one million people died during the Great Famine from starvation or from typhus and other famine-related diseases. An estimated two million more emigrated from the country. India was hit by recurrent famine from 1760 AD to till 1943 AD. As per British sources, there were more than 85 million Indians died in these famines which were in reality genocides done by the British Raj. But after the green revolution the bad effects of famine have not been seen in India. Famines, genocides and plagues where the main reasons why Indian sub continent had a population of only 200 million people in the 1800s which has increased to 1,781 million in the present time.