'Sickness' is perfectly accepable. It seems that the two words disease and sickness, are interchangeable in everyday speech.
Inserting the word 'mentally' (NWT, 1 Tim. 6:4), is taking a bit of a liberty with translation. But then to blanket apply 'mentally diseased' to all who disagree or who leave the JWs is offensive and stupid.
Miriam Webster: Sickness (noun):
1. Impairment of normal physiological function affecting part or all of an organism.[Wordnet]
2. Defectiveness or unsoundness; "drugs have become a sickness they cannot cure"; "a great sickness of his judgment".[Wordnet]
3. The state that precedes vomiting.[Wordnet]
4. The quality or state of being sick or diseased; illness; sisease or malady.[Websters]
5. Nausea; qualmishness; as, sickness of stomach.[Websters].
Miriam Webster: Disease (noun):
1. An impairment of health or a condition of abnormal functioning.[Wordnet]
2. Lack of ease; uneasiness; trouble; vexation; disquiet.[Websters]
3. An alteration in the state of the body or of some of its organs, interrupting or disturbing the performance of the vital functions, and causing or threatening pain and weakness; malady; affection; illness; sickness; disorder; -- applied figuratively to the mind, to the moral character and habits, to institutions, the state, etc.[Websters].
Miriam Webster: Disease (verb):
1. To deprive of ease; to disquiet; to trouble; to distress.[Websters]
2. To derange the vital functions of; to afflict with disease or sickness; to disorder; -- used almost exclusively in the participle diseased.[Websters]
3. Base verb from the following inflections: diseasing, diseased, diseases, diseaser, diseasers, diseasingly and diseasedly.[Eve - graph theoretic]
Vine's
<A-3,Noun,3554,nosos>
akin to Lat. nocere, "to injure" (Eng., "nosology"), is the regular word for "disease, sickness," Matt. 4:23; 8:17; 9:35; 10:1, RV, "disease," AV, "sickness;" in Matt. 4:24; Mark 1:34; Luke 4:40; 6:17; 9:1; Acts 19:12, AV and RV render it "diseases." In Luke 7:21, AV has "infirmities." The most authentic mss. omit the word in Mark 3:15. See SICKNESS.
Other translations on 1 Timothy 6:4
New American Standard Bible (©1995)
he is conceited and understands nothing; but he has a morbid interest in controversial questions and disputes about words, out of which arise envy, strife, abusive language, evil suspicions,
King James Bible
He is proud, knowing nothing, but doting about questions and strifes of words, whereof cometh envy, strife, railings, evil surmisings,
Douay-Rheims Bible
He is proud, knowing nothing, but sick about questions and strifes of words; from which arise envies, contentions, blasphemies, evil suspicions,
AM