Taken from, Can Jehovah's Witnesses Survive? by Trevor Willis (Formating not preserved)
How can we protect ourselves against false teachers? The Bible's counsel regarding how to deal with them is clear. (Read Romans 16:17; 2 John 9-11.) ‘Avoid them,’ says God's Word. Other translations render that phrase ‘turn away from them,’ ‘keep away from them,’ and ‘stay away from them!’ There is nothing ambiguous about that inspired counsel. Suppose that a doctor told you to avoid contact with someone who is infected with a contagious, deadly disease. You would know what the doctor means, and you would strictly heed his warning. Well, apostates are ‘mentally diseased,’ and they seek to infect others with their disloyal teachings. (1 Tim. 6:3) Will you heed Jehovah's warnings? The Watchtower - 15 July 2011
The notion that members who leave are mentally diseased was based on a scripture, from their own translation:
If any man teaches other doctrine and does not assent to healthful words, those of our Lord Jesus Christ, nor to the teaching that accords with godly devotion, he is puffed up [with pride] not understanding anything, but being mentally diseased over questionings and debates about words. 1 Timothy 6:3, 4 - New World Translation
Other translations use terms such as: morbid interest, unhealthy interest, unhealthy craving