You see the believers, in their mercy, love and compassion for one another, as a single body; when one limb is ailing, the rest of the body joins it in sleeplessness and fever.
ترى المؤمنين في تراحمهم وتوادهم وتعاطفهم كمثل الجسد إذا اشتكى عضوا تداعى له سائر جسده بالسهر والحمى
Tera'l-mü'minîne fî terâhumihim ve tevâddihim ve teâtufihim ke-meseli'l-cesed; ize'ştekâ uzven tedââ lehû sâiru cesedihî bi's-seheri ve'l-hummâ.
Bukhari, Adab, no. 6011 (from Nu'man ibn Bashir).
The comparison is drawn from an everyday experience: a person whose finger aches cannot sleep that night.
The place that hurts is small; it is the whole body that stays awake.
The hadith describes the bond among believers by this measure and names three things: mercy, love, compassion.
A community that stays indifferent to pain in a distant city is, by this description, a numbed body.
Here, fever and sleeplessness are signs of health.