Sporadic is an adjective that you can use to refer to something that happens or appears often, but not constantly or regularly. The mail carrier comes every day but the plumber's visits are sporadic––he comes as needed.
A specialized use of sporadic is to describe a disease that appears only occasionally in random cases, and is therefore not an epidemic. English borrowed the word sporadic from Greek sporadikós "scattered." A very near synonym is intermittent.