Even Iago from the beginning of the play describes Othello as a trusting man "who thinks men honest that but seem to be so." Othello's trusting nature is perhaps his greatest vulnerability, and it is this flaw that makes him so susceptible to Iago's manipulations. At the end of the play, Othello rightly says of himself that he was a man "not easily jealous" (5.2). Othello is not naturally suspicious or jealous, but he was manipulated to become suspicious because he trusted Iago, who he thought was his right-hand man.
However, Iago is much more naturally suspicious of others, and he does not need any "ocular proof" to be that way. Although he has no evidence at all of the fact, he suspects that both Othello and Cassio have slept with his wife:
I know not if 't be rue,
But I for mere suspicion in that kind
Will do as if for surety. (1.1)
Later in Act 2, Iago calls his jealousy of Othello "a posionous mineral" that is eating away at him. Iago is much like the kind of jealous, or suspicious man, that Emilia describes,
They are never jealous for the cause,
They are jealous for they are jealous. (3.3)