word usage - Is there any difference between being ill and sick?

I can say I'm ill or I'm sick. But what is the difference between the usage of these terms?

I've heard that one can use sick for longer-term and ill for shorter-term, but is that really correct? How are these terms different for native speakers?

Answer

While those might mean the same for the laymen, from a medical point of view, there is a difference between illness and sickness.

Medical sociology has long made the distinction between illness and sickness. Illness is the objective diagnosis that an external impartial observer is able to make based on the constellation of symptoms which the patient presents. Sickness is the social role that the patient adopts as the patient and other concerned stakeholders, in relationship with the patient, interpret the meaning of the illness.

From what I get of it, someone might see themselves as sick (with the social/role aspect of it) but not actually be ill (in a medical sense). Also, this paper might provide some useful reading.