The only known counter-spell is sacrificial protection, which uses themagic of love. However, one may dodge the green bolt or block it with a physical barrier. The Killing Curse, as an "unblockable" Curse, cannot be intercepted by another spell, except in circumstances of Priori Incantatem, where the caster and his opponent's wands and spells are locked together. An explosion or green fire may result if the spell hits something other than a living target.
Only two wizards are known to have survived blows from this deadly curse: Harry Potter and Tom Riddle. Harry survived two direct attacks: once in 1981 after his mother's self-sacrificing love protected him from Lord Voldemort, and once in 1998 after the curse, cast again by Voldemort, failed to kill Harry, as he was tethered to life by Voldemort himself, due to Lily's blood protection which he took inside himself during his rebirth. Lord Voldemort remained alive after the aforementioned curse from 1981 rebounded and struck him because of his Horcruxes.Voldemort was notorious for using this curse regularly and indiscriminately.
Along with the Cruciatus and Imperius curses, the Killing Curse is known as one of the most terrible curses in the wizarding world. After the Wizards' Council was reformed into the Ministry of Magictighter restrictions were placed on the use of certain kinds of magic. The Killing Curse was deemed by the Ministry to be Dark magic, and, along with the Cruciatus and Imperius curses, were declared "unforgivable" in 1717, with the Killing Curse considered to be the most deadly of the three. Cursing another human with the Killing Curse would carry the punishment of a life sentence in Azkaban.
However, the First Wizarding War, when Barty Crouch Sr. was in charge of the Department of Magical Law Enforcement, he fought violence with violence, legalising the three Unforgivable Curses for Aurors against the Death Eaters in order to win the war. This was repealed once the war was over as it was no longer necessary, though, in the 1994–1995 school year, Barty Crouch Jr. (under the disguise of Alastor Moody) showed these three curses to his fourth year classes on spiders despite the Ministry's disapproval.
When Lord Voldemort took over the Ministry, the three curses were once again legalised: this time every wizard and witch had the right to use them as they please. In fact, they were practiced in Hogwarts as part of the curriculum ofDark Arts class under the tutelage of Professor Amycus Carrow. After Voldemort's death and the revolutionising of the Ministry under Minister Kingsley Shacklebolt, the three curses were once again forbidden.
It should be noted that despite the curse being illegal, references have been made to Aurors using deadly force against opponents, though whether this means they were authorized to use the killing curse specifically is unclear. Despite the circumstances, it is unknown whether the Killing Curse was used by anyone but Voldemort and his Death Eaters during the Battle of Hogwarts. It is also known that use of this curse may go unpunished if there is sufficient evidence that the caster did so under the influence of the Imperius Curse.