Worm_Sohanad.JH may be dropped by WORM_AUTORUN.DIO. It may be downloaded from certain remote sites.

It sends copies of itself to target recipients using certain instant messaging applications. It sends message(s) via the instant messaging application Yahoo! Messenger.

It accesses Web sites to download files detected as WORM_AUTORUN.DIO and TROJ_DROPPER.HUJ. As a result, malicious routines of the downloaded files are exhibited on the affected system.

Technical details can be found at this Trend Micro page.