This article uses an in-universe perspective.
The humans that become infected will feel weak at first, but after approximately 12 hours they will no longer feel but will become extremely hungry. They will see uninfected people as monsters much the same as they saw the infected before. They will feel stronger than ever before and will have increased speed and reflexes. They will see other infected as if they were normal people. They also retain emotions and even pair up as can be seen with the alpha male and alpha female (the infected that is caught and experimented upon) and with Vatsala and Pritam. They also retain enough intelligence to devise traps that they have previously seen. They lose all of their hair after a time and become photosensitive to the point that their skin will literally burn when exposed to sunlight.These creatures are known as Darkseekers.
The transformation happens much more quickly than in humans. As can be seen with Sam, the transformation of dogs can take as little as an hour. The only known animals to be infected are dogs and rats. It is believed that lions, deer, and birds may be immune. They are seen in daylight (apparently unaffected by the virus). As much eaiser it is for an animal to turn than a human, it is also much faster to kill an infected animal than an infected human.
The cure is the reason Robert Neville never left New York. It was "ground zero." He spent years with his dog Sam testing on infected rats, keeping himself sane by various methods. After several different variations of his own blood had created, he finally came up with one that did not kill the infected host rat. When he captured a darkseeker to test on, it killed her and he had to administer adrenaline to save it. However, when he tried slowing down the metabolism of the infected woman by surrounding her with ice, her breathing slowed, she became less aggressive, and her skin gained back color. The cure was given to Anna, the human who recently found him. She takes it to a survivors colony in Vermont, where they should be able to do a chemical anaylasis of the cure and replicate more from their own immunity in their blood.