To spread an update, Stuxnet installed a file-sharing server and client on each infected machine, and machines that were on the same local network could then contact one another to compare notes about the version of Stuxnet they carried; if one machine had a newer version, it would update the others. To update all the machines on a local network, the attackers would have only had to introduce an update to one of them, and the others would grab it.
Also clever: auto-updates for malware.
