Elf Infected are the new reskin of the previous zombies of Apocalypse Rising. This was done during the 2014 Holiday Update. The Elf Infected reskin was removed after the 2014 Holiday Update.
Besides the obvious clothing, the elf infected produce a jingle sound whenever they move. If they manage to attack the player, a "ho ho ho" sound is heard. That is the only thing changed about the elf.