Does Orochimaru need a host? After the Hokage the third took his arms, Orochimaru needed a new body for himself. Later, shortly before Sasuke killed him, this body was rejecting him, so he wanted to take Sasukes body as his new host.
In his fight against Sasuke, he abandons his body and appears as a huge snake. And here's the question: Does Orochimaru need a host at all, and why? Or can he stay as a snake for longer time?
Do you remember when Orochimaru had been discovered by the Third Hokage while he was experimenting on people1? That's what he was researching: a technique for immortality. 

It's called 不屍転生 (Fushi Tensei) or literally Living Corpse Reincarnation and allows Orochimaru to transfer his soul into his  prey's body and take possession of it.

He researches this because as he says, he wants to learn all the existing techniques, but since that is impossible for a person with a normal lifespan, he develops this technique in order to have more time.

The problem with this technique is that each host lasts around 3 years, so he needs to find another one after that. If he could find a perfect container he could stay in it for a full lifetime, but he would need to find another one again anyways after that.

1: Chapter 122 in Naruto, Volume 14