What sort of curse does unicorn's blood give?

Question

With regards to drinking unicorns blood, according to Firenze:

you will have but a half-life, a cursed life, from the moment the blood touches your lips

How is this curse played out? Is it the initial drinking of it that bestows a curse, or is the curse that you must keep drinking the blood to survive?

Slytherincess mentions in an answer

Unicorn blood is an ingredient in Voldemort's initial resurrection potion which allows him to return to the infant form that Wormtail carries around, that we see in the beginning of Goblet of Fire

Is this the point at which the curse is bestowed? Lifted? If the Latter is the curse to be stuck on the back of someone's head?

Is there a timeline of Voldemort's unicorn drinking 'habit' that clears up any of my questions? E.G Time of first drinking, effect, time of second drinking etc?

Further, is Quirrel equally cursed? Or is he alone cursed?

Answer 1

Philosopher's Stone has a lot to say on this subject.

You have slain something pure and defenceless to save yourself and you will have but a half life, a cursed life, from the moment the blood touches your lips.’

Philosopher's Stone - page 188 - UK - chapter 15, The Forbidden Forest

So each time the unicorn's blood touched Quirrell's lips, the curse of a half life, a cursed life, would be enacted. My impression is that it only takes one time drinking unicorn blood to invoke a life-long curse, unlike a spell which wears off after a given period of time. I would imagine the curse is lifted at the time of the cursed person's death. No, the curse from drinking unicorn blood does not cause the head of the person possessing you to grow out of the back of your own head -- remember, Voldemort possessed Quirrell's body all the way back in September, maybe even August, of 1991 (HP LEXICON - TIMELINE).

Harry spotted Professor Quirrell, too, the nervous young man from the Leaky Cauldron. He was looking very peculiar in a large purple turban.

Philosopher's Stone - page 91 - UK - chapter 7, The Sorting Hat

As far as a timeline for drinking the unicorn blood goes, during Harry's first year (1991-1992) Harry, Hermione, Neville, and Draco serve detention with Hagrid on or around May 26, 1992. Harry and Draco came across the carcass of a unicorn in the Forbidden Forest, and then encountered Voldemort (in Quirrell's body) drinking the unicorn's blood [HP LEXICON - TIMELINE].

The cloaked figure reached the unicorn, it lowered its head over the wound in the animal’s side, and began to drink its blood.

‘AAAAAAAAAAARGH!’

Malfoy let out a terrible scream and bolted¹ – so did Fang. The hooded figure raised its head and looked right at Harry – unicorn blood was dribbling down its front.

Philosopher's Stone - page 187 - UK - chapter 15, The Forbidden Forest

The other slain unicorn was found the Wednesday before, within a week of the Forbidden Forest escapade recounted above:

‘Look there,’ said Hagrid, ‘see that stuff shinin’ on the ground? Silvery stuff? That’s unicorn blood. There’s a unicorn in there bin hurt badly by summat. This is the second time in a week. I found one dead last Wednesday. We’re gonna try an’ find the poor thing. We might have ter put it out of its misery.’

Philosopher's Stone - page 183 - UK - chapter 15, The Forbidden Forest

There are no other instances of Voldemort drinking unicorn blood in Philosopher's Stone, although he indicates he drank it over the course of "weeks" via Quirrell and the effects were strengthening:

‘See what I have become?’ the face said. ‘Mere shadow and vapour ... I have form only when I can share another’s body ... but there have always been those willing to let me into their hearts and minds ... Unicorn blood has strengthened me, these past weeks ... you saw faithful Quirrell drinking it for me in the Forest ...'

Philosopher's Stone - page 213 - UK - chapter 17, The Man With Two Faces

Your last question about Quirrell being equally as cursed as Voldemort is a great one, because according to J.K. Rowling, Quirrell was completely subjugated by Voldemort, and although he attempted at times to resist Voldemort, Voldemort was too powerful and always retained control.

That said, Quirrell isn't innocent. See Pottermore info on Quirrell [HERE] and [HERE]. Quirrell sought Voldemort out with a naive arrogance, not understanding how dangerous Voldemort was, even in spectral form.

So was Quirrell cursed as well as Voldemort? He certainly didn't live much longer after he drank the unicorn blood.

ETA: Regarding how the potion with the unicorn blood reacted when Voldemort took it, I cannot give a definitive answer. We know that drinking unicorn blood will give a person "a cursed life, half a life" but we don't know if brewing unicorn blood (heating it) or blending it with snake venom changes its composition enough to bypass the curse. There's just not any canon information about this; it's not addressed. I could speculate, but it wouldn't be canon.

Regarding Voldemort's face on the back of Quirrell's head, there's nothing to indicate that unicorn blood had anything to do with this. Voldemort had possessed Quirrell at least by September 1, 1991, and Quirrell was wearing his turban to hide Voldemort's face by that date. The earliest we know for sure that Voldemort was drinking unicorn blood is on or around May 19, 1992, a full 8.5 months later. And because Quirrell was killed, and Voldemort abandoned Quirrell's body as a host, we don't know if the face would've lasted infinitely.

As for the "half a life" issue, it's either literal or metaphorical. If it's literal, then Voldemort should have lived for an additional 70 or so years past the date that he actually died (May 2, 1998), for he was approximately 70 when he died. This would be without Horcruxes, of course. If it's metaphorical, then it suggests Voldemort's life, as Rob said in his comment, wouldn't be fulfilling. Kind of like A life lived in fear is a life half lived².

¹_{Malfoy FTW!}
²_{From the movie Strictly Ballroom}

Answer 2

This is entirely speculative, but it could be similar to, or even in reference to, the half-life of a radioactive material.

Just a quick physics lesson:

Half-life is how the lifetime of a radioactive material is measured.

Let's say a radioactive material has the half-life of a year. However much radiation that material emits, in a year it will have decayed sufficiently so that it will only emit half as much. A year on from that it will have halved again, so would now only be a quarter of what it was 2 years ago. After 3 years it would be 1/8th of the original radioactivity, and so on forever.

This means that radioactive materials are always and will always be radioactive, but the radiation they emit decreases over time. So the higher the radioactivity, the faster it decays. And lower radioactivity means it decays slower.

This could be similar to what happens when a human drinks the blood of a unicorn.

Once they drink the blood, they will have the ability to live forever (not die of natural causes, but can still be killed with ). However after every half-life, let's stick with around a year for a human as a guess (probably different for different creatures: humans, centaurs, giants etc.), they will be half as strong as they were the year before. This will carry on forever, without the person ever dying but simply getting weaker and weaker, year after year.

There is a lot of evidence for this.

For example, Firenze himself, who knows of such things, says:

You have slain something pure and defenceless to save yourself and you will have but a half life, a cursed life, from the moment the blood touches your lips.

Philosopher's Stone - page 188 - UK - chapter 15, The Forbidden Forest

Rather than saying a half life, a cursed life, and meaning them as two different ways of saying the same thing, he could be saying that the half-life is the cursed life you must endure.

Perhaps Quirrell was close to dying after Voldemort had been draining strength from him for so long, forcing Quirrell to search out the Unicorn to kill in order to sustain his own life, so that Voldemort could carry on leeching life force from him.

This would explain why it seems Voldemort doesn't have the negative side effects from drinking Unicorn blood once he has been fully resurrected, because he never drinks the blood, Quirrell does. Voldemort simply leeches the increased life-force from him.

It would also explain why he keeps going back to the forest to drink the Unicorn blood.

As Firenze said, once the blood touches your lips you will from then on live a cursed life. So why keep going back once you are already cursed? It couldn't be to sustain the curse, the curse is permanent.

It is likely that each time the blood is drank, the drinker gains more strength from it, just like increasing the base radioactivity of a material.

Whilst the half-life would be the same, the amount of strength the person has would be greater, making it longer for them to reach a weak and feeble state.

This would make sense, considering Quirrell is so much stronger closer to the end of the book. Perhaps he needed this strength in order to endure the trials to reach the Mirror of Erised, which would explain why he had not bested them earlier and had to wait until the end of the year before he was able to succeed.

As I said earlier, this is all speculation, and not based on canon, but there is apparently strong evidence for this theory.