The Compendium of Caraka 2.6.12_2

2.6.12_2

taṃ sarvarogāṇāṃ kaṣṭatamatvādrājayakṣmāṇamācakṣate bhiṣajaḥ; yasmādvā pūrvamāsīdbhagavataḥ somasyoḍurājasya tasmādrājayakṣmeti//

Physicians call it rājayakṣmā because it is the most dreadful of all diseases; or because it originally afflicted the divine Soma, king of the stars, therefore it is called rājayakṣmā.