The illness has reached the bone marrow; (fig.) to be seriously ill and beyond cure; a situation is critical and beyond remedy, deeply entrenched and hard to change.
长期以来形成的不良习惯已经病入骨髓，要想彻底改掉需要很大的毅力。
The long-standing bad habits have become deeply ingrained, and it will take great perseverance to completely get rid of them.
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