If you peruse the various vendors catalogs of malware detected, successfully cleaned malware is re-released continuously with tweaks or is polymorphic in a manner that requires updates in detection to take care of software updates their creators apply.
Viruses and malware are just like OS systems now. All those security updates that you see released for Linux, Windows, Mac OS? The more sophisticated malware gets for a better want of term, "virility patches". They don't get cleaned, they just get new features so they can reinfect lax computer owners computers who think that one "Cleaning" is sufficient to make the problem go away.
Also like OS systems, they can be very modular in nature and have their own "Programs and Features" reconfigurability as well.
One trick pony malware like Code Red or Love Bug likely will never be re-released as they were simple hacks for specific vulnerabilities. What's out there now is persistent and always available.