From a widespread plague to the bite of a dire rat, disease is a serious threat to common folk and adventurers alike. Diseases rarely have a limited frequency, but most have a lengthy onset time. This onset time can also be variable. Most diseases can be cured by a number of consecutive saving throws or by spells such as remove disease.
Note: Not everything listed here is a true "disease" as some of the below are more accurately described as infections, parasites, or other unfortunate maladies adventurers might occasionally have to cope with.