In Act IV of Macbeth, Macbeth receives new prophecies from the witches (in reality, the apparitions). The new prophecies are as follows:
Macbeth! Macbeth! Macbeth! beware Macduff;
Beware the thane of Fife. Dismiss me. Enough.
Be bloody, bold, and resolute; laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth.
Macbeth shall never vanquish'd be until
Great Birnam wood to high Dunsinane hill
Shall come against him.
For Macbeth, the single most dangerous prophecy is the one regarding Macduff.
Macbeth does not fear the other prophecies based upon the fact that all men are born of women (or so Macbeth thinks--he has forgotten about Cesarean sections) and that trees cannot pull their roots from the ground and move.
Therefore, the only prophecy that threatens Macbeth is the one which states Macduff to be an immediate enemy. One can be more certain that Macbeth is fearful of this prophecy because he does not deny its ability to "happen" (like he does the other two).