Without effective human intervention, epidemics and pandemics typically end only when the virus or bacteria has infected every available host and all have either died or become immune to the disease.
Bacteria are single-celled organisms. Bacteria are the model organisms for everything that we know in higher organisms. There are 10 times more bacterial cells in you or on you than human cells.
Bacteria live in unbelievable mixtures of hundreds or thousands of species. Like on your teeth. There are 600 species of bacteria on your teeth every morning.