Avalanche is a large nasty botnet, which was just disabled by a large
coordinated action by industry and law enforcement in multiple
countries. It was a lot of work, involving among other things
disabling or sinkholing 800,000 domain names used to control it.
More info here:
As both items point out, if your users are infected with Avalance,
they're still infected, but now if you disinfect them, they won't get
reinfected. At least not with that particular flavor of malware.