Origin: Germanic Old English suffix
Sickness has 4 different meanings across 1 category:
impairment of normal physiological function affecting part or all of an organism
"The sudden sickness that struck the colony was caused by a virus impairing the normal physiological functions of their respiratory systems."
defectiveness or unsoundness
"drugs have become a sickness they cannot cure"
"a great sickness of his judgment"
The quality or state of being sick or diseased; illness.
"The sudden sickness caused him to rush outside and throw up immediately after eating spoiled food."
In plain English: Sickness is when your body feels unwell and you might get sick from germs, food poisoning, or other health problems.
"She stayed home from work because she had to deal with her sudden sickness."
Usage: Use "sickness" to refer generally to the condition of being ill, though it is often less common than simply saying someone has an "illness." Avoid using it as a direct synonym for specific diseases like flu or cold unless emphasizing the general state of unwellness rather than the particular diagnosis.
The word sickness comes from the Old English term sēocnes, which originally described a state of being ill or infirm. It entered modern usage through Middle English as sikness before evolving into its current form by combining "sick" with the suffix "-ness."