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?
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.
The formal range of meanings for each word is more or less the same, but they carry different connotations and usage. It may vary from region to region, but in the USA, it is fairly common to use ill for longer or more serious issues, like cancer, and sick for more immediate things, like the nausea involved in cancer treatment.
Additionally, sick is used in some idiomatic expressions where ill would not fit native sensibility.
“I am sick and tired of X,” is used to mean that somebody's patience is worn out. No native speaker would ever say ill and tired in this case. Likewise, if someone were to drink too much and vomit, one would say, “He got sick.” To get sick is so strongly connected with vomiting that you can even say, “He got sick on his shoes,” or “She got sick last night,” for instance and there will be no ambiguity among native speakers in the USA.
Likewise, to fall ill is never worded to fall sick. To us, that would be just odd.
Either word might be used to describe someone’s mental illness, such as “He is sick in the head,” or “He is mentally ill,” though the phrase “mental illness” sounds right to us, and you will probably not often hear an American, at least, use the exact phrase “mental sickness.”
Illness refers to a medical condition.
Sickness refers to the way one feels.
Illness often makes one feel sick, so the terms are often used interchangeably in colloquial speech.
But, one can be ill without being (feeling) sick. Likewise, one might feel sick after, say, seeing blood, without being ill.
A related Australianism.
Having a day off work - for the stated reason of being too ill to attend - is known as chucking a sickie. You would never call it 'chucking an illie'.
To the question: the biggest difference is for definitions involving nausea in which case sick is used exclusively. Hence sicking up.
Additionally, the colloquial meanings (ie not strictly related to being unwell) tend to use sick. For example, sick and tired (see Ryan's ans.); fully sick (meaning really good); sick to death (worse than sick and tired); you make me sick (you are disgusting, obnoxious or offensive to my sensibilities).
In Indian context both can be used interchangeably. But there is a subtle difference between the two. Sick could be used if someone is annoyed by one's act or behavior. He'd be rather sick than feeling illness by the deeds of that person. Likewise if someone has done something wrong to me, then I'd be feeling sick.
Whereas ill means that a person has been acquired by the disease. So it might be possible that, the person might be feeling sick(feeling frustration due to suffering). So this is the difference between the two. But I've seen that generally people conversing in colloquial language uses the both the terms interchangeably.