Morbid is sometimes used interchangeably with diseased, but is commonly applied, in a somewhat technical sense, to cases of a prolonged nature; as, a morbid condition of the nervous system; a morbid sensibility, etc.

Webster's Revised Unabridged Dictionary (1913)

Example sentences

sick The doctor sat up all night with the sick old man.
unhealthy If you don’t do more exercise, you’ll be unhealthy.
morbid He has a morbid fondness for murder mysteries.
sickly Since childhood, Boldi has been a very sickly boy.
infected Tom and his workers killed all the infected sheep.

Popularity over time

Word	1850 → 2019	Trend
diseased		falling
sick		rising
unhealthy		rising
morbid		rising
sickly		rising
infected		steady

Use in printed books, per decade from 1850 to 2019.

By length

3 letters: ill¹
4 letters: sick¹
5 letters: mesel
6 letters: morbid², ailing², unwell², sickly², infirm²
7 letters: unsound²
8 letters: infected³
9 letters: unhealthy³
10 letters: pathologic⁴
11 letters: distempered
12 letters: pathological⁵

The small number after each word is its number of syllables (morbid² = 2 syllables).