Final answer:
Secular trends refer to gradual changes in disease frequency over long periods, differing from epidemics and pandemics, with endemic diseases being consistently present in a population.
Step-by-step explanation:
Gradual changes in the frequency of disease over long time periods refer to secular trends. These trends are different from sudden increases in disease occurrence, known as epidemics, which are limited to certain regions. On the other hand, when epidemics spread across multiple countries or globally, they become known as pandemics. Endemic diseases are those that are consistently present at a steady rate within a given population. For example, malaria is endemic in certain tropical countries where it is transmitted year-round, whereas influenza might be epidemic in certain winters if there are unexpectedly high numbers of cases.