Macbeth speaks this line -- "Infected be the air whereon they ride,/and damned all those that trust them!" -- about the witches.

This is complete irony. What the sentence means is that the witches are such terrible, sick creatures that even the air which they ride on becomes infected from them. And he is damning (wishing them to go to hell) whoever trusts the witches. This is ironic because Macbeth himself has trusted the witches.

Macbeth, along with General Banquo, came across the three witches in the beginning of the play. The witches made prophecies about both of them. Although Macbeth did not believe them at first, he did believe them later when one of the predictions the witches made came true. They said that Macbeth would be given a high rank and he was given a high rank (he became Thane of Cawdor).

So by damning those who trust the witches, he is in fact damning himself.

What this may suggest about Macbeth is that he deeply regrets having trusted the witches. He is experiencing so much regret about his decisions and their consequences that he even wishes himself ill as a result. One might also interpret the fact that he wishes damnation on those who trust witches to suggest that he has faith in the afterlife, judgement from a higher power, and that everyone eventually faces the consequences of their deeds.

Yet, he has already succomb to their seduction; not only the air has been infected by him, but so has he.  In fact, an early definition of "evil" still current during Shakespeare's time was that of an infection, in someways growing out of the understanding of the black plague as an evil that spread.  "Damned all those that trust them" is ironic because he damns himself but more out of frustration than anything else because he cannot control them in the way they have a control over him.  Just as he wants more information, "poof"! they vanish. Yet of course the other irony is he is indeed "damned" for trusting them for more information for him to realize his now pernicious dreams of power.

This is a response Macbeth has to what the witches have told him. He is basically saying that he trusted their message and therefore he himself is "damned." He says that those who believe the witches will be damned. Because Macbeth failed to consider their warning, it shows his own arrogance.