In the beginning of Harry Potter and the Philosopher's Stone, Harry meets Professor Quirrell in The Leaky Cauldron for the first time, and the two shake hands with no problem:
A pale young man made his way forward, very nervously. One of his eyes was twitching.
‘Professor Quirrell!’ said Hagrid. ‘Harry, Professor Quirrell will be one of your teachers at Hogwarts.’
‘P-P-Potter,’ stammered Professor Quirrell, grasping Harry’s hand, ‘c-can’t t-tell you how p-pleased I am to meet you.’
Philosopher's Stone - page 55 - Bloomsbury - chapter 5, Diagon Alley
And, as we know, by the end of Philosopher's Stone, Harry's touch is enough to kill Quirrell.
Quirrell raised his hand to perform a deadly curse, but Harry, by instinct, reached up and grabbed Quirrell’s face –
Quirrell rolled off him, his face blistering too, and then Harry knew: Quirrell couldn’t touch his bare skin, not without suffering terrible pain – his only chance was to keep hold of Quirrell, keep him in enough pain to stop him doing a curse.
Philosopher's Stone - pages 213-214 - Bloomsbury - chapter 17, The Man with Two Faces
Why could Quirrell touch Harry at the beginning of Philosopher's Stone without pain, but by the end of the story Harry's touch was lethal to Quirrell? Had Voldemort not taken possession of Quirrell's body by the time of Quirrell and Harry's meeting at The Leaky Cauldron?