What improvements in Europe in the 18th century contributed to the decline of disease?
The major cause of the decline in epidemics came from improvements in what we would call "public health." Public health refers to things like public sanitation that influence the health of all the people in a society.
In the 1700s, many places in Europe came to have better sanitation. Improvements in sanitation meant that diseases that were caused by germs getting into drinking water, for example, became less common.
Not all of the decline of famine can really be linked to human causes. For example, we do not exactly know why the Black Death stopped being such a problem. This improvement was not caused by any known human action.