Are there any differences when those words are used? By whom they are used?
All English
English fiction:
I would guess that "illness" is rather a term which is used in spoken language and "disease" is more formal as both seem to be equally common in fiction, but I am not sure about it.
The following diagram is from the University of Ottawa, "Society, the Individual, and Medicine (SIM) curriculum" notes. The diagram illustrates the difference between illness, disease, and sickness, and it includes brief descriptions.
The words overlap in their semantic field and can be used interchangeably, but the connotations of the two words vary.
The etymology of illness indicates the malevolent nature of the physical condition:
"disease, sickness," 1680s, from ill + -ness. Earlier it meant "bad moral quality" (c.1500).
c.1200, "morally evil" (other 13c. senses were "malevolent, hurtful, unfortunate, difficult"),
from Old Norse illr "ill, bad," of unknown origin.
Not considered to be related to evil.
Main modern sense of "sick, unhealthy, unwell" is first recorded mid-15c., probably related to Old Norse idiom "it is bad to me." Slang inverted sense of "very good, cool" is 1980s. As a noun, "something evil," from mid-13c.
The etymology of disease indicates the discomfort and inconvenience of the physical condition:
early 14c., "discomfort, inconvenience,"
from Old French desaise "lack, want; discomfort, distress; trouble, misfortune; disease, sickness," from des- "without, away" (see dis-) + aise "ease" (see ease).
[c.1200, "physical comfort, undisturbed state of the body; tranquility, peace of mind," from Old French aise "comfort, pleasure, well-being; opportunity," which is of unknown origin, despite attempts to link it to various Latin verbs; perhaps Celtic. According to OED, the earliest senses in French appear to be 1. "elbow-room" (from an 11th century Hebrew-French glossary) and 2. "opportunity." This led Sophus Bugge to suggest an origin in Vulgar Latin asa, a shortened form of Latin ansa "handle," which could be used in the figurative sense of "opportunity, occasion," as well as being a possible synonym for "elbow," because Latin ansatus "furnished with handles" also was used to mean "having the arms akimbo." OED editors add, "This is not very satisfactory, but it does not appear that any equally plausible alternative has yet been proposed."]
Sense of "sickness, illness" in English first recorded late 14c.; the word still sometimes was used in its literal sense early 17c.
By constant use the malevolent nature and the discomfort of sickness have commingled in the two words illness and disease.
According to Wikipedia:
In many cases, terms such as disease, disorder, morbidity and illness are used interchangeably.3 There are situations however when specific terms are considered preferable.
Disease
The term disease broadly refers to any condition that impairs the normal functioning of the body. For this reason, diseases are associated with dysfunctioning of the body's normal homeostatic process.4 Commonly, the term disease is used to refer specifically to infectious diseases, which are clinically evident diseases that result from the presence of pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular organisms, and aberrant proteins known as prions. An infection that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal bacteria and yeasts in the gut, or of a passenger virus, is not considered a disease. By contrast, an infection that is asymptomatic during its incubation period, but expected to produce symptoms later, is usually considered a disease. Non-infectious diseases are all other diseases, including most forms of cancer, heart disease, and genetic disease.
Illness
Illness and sickness are generally used as synonyms for disease.5 However, this term is occasionally used to refer specifically to the patient's personal experience of his or her disease.[6][7] In this model, it is possible for a person to have a disease without being ill (to have an objectively definable, but asymptomatic, medical condition), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or medicalizes a non-disease situation in his or her life). Illness is often not due to infection, but a collection of evolved responses—sickness behavior by the body—that helps clear infection. Such aspects of illness can include lethargy, depression, anorexia, sleepiness, hyperalgesia, and inability to concentrate.[8][9][10]
Interestingly, the current use of the words, reverses the etymological implications with disease representing the objective nature, and illness representing the subjective experience of a malady.
etymonline.com wikipedia.com
The difference is less in their actual meaning and more in their purported usage. In olden days, any ailment was a disease, but now 'known' or 'easily recoverable' ailment is considered illness and those that required severe, life-death type treatment are considered diseases. So 'fever' is considered illness, while 'malaria' or 'jaundice' is disease. There is no bacterial/viral distinction as well as the words are used for either.
We see that Oald says the words illness and disease are synonyms, the University of Ottowa wants to see semantic differences, but I am sure that such distinctions are unknown to almost any speaker. My experience is that illness is preferred in spoken language and disease is often used in texts that have medical topics such as Alzheimer's disease. But you find illness in such texts as well.
