Today is the feast of St. Roberto Bellarmino, polemical theologian and Doctor of the Church. Even if you didn't know he was a Jesuit you could figure it out by the fact that so many legends, apocryphal stories, and outright falsehoods have collected around him.
David Hume has an interesting comment about Bellarmine in The Natural History of Religion:
Bellarmine patiently and humbly allowed the fleas and other odious vermin to prey upon him. "We shall have heaven," said he, "to reward us for our sufferings; but these poor creatures have nothing but the enjoyment of the present life."
Like a number of Hume's weirder claims this comes from Pierre Bayle, who as a Calvinist is not wholly the most reliable source for information about Catholic saints. The advantage of Bayle, though, is that he tells us where he's getting these things. Bayle is drawing the story from Jacopo Fuligatti's life of Bellarmine. Fuligatti gives three stories about insects and Bellarmine's patience: first, it's said he counted gnats and other minor vexations to be from God, so he wouldn't drive them away, but simply endured them patiently; second, that he was once bitten by insects at Mass and turned to the statue of Christ to pray; and third, that Cardinal Crescentius told the story that in Rome, where flies are a problem, he wouldn't drive them away, and when asked why said, "It is unjust to disturb those little creatures whose only paradise is the freedom of flying and landing where they wish." Bayle sums it up by saying that he was said to be so patient that he allowed flies and other small insects to be troublesome to him; and being a sarcastic Calvinist, he insists on explicitly saying that shooing away bugs is consistent with the teachings of Christ. It should not need to be said, but unfortunately probably does, that there is nothing whatsoever in Bellarmine to suggest that he would not agree to this; that the stories are second-hand (third-hand by the time we get to Bayle) and, except for the third, which alone is sourced, they are vague and generic moral tales about patience and suffering which have parallels elsewhere in hagiographical convention (the first story explicitly links the bugs to minor vexations of life; the second links them to hell); that if Bellarmine actually did make the comment in the third story, it might well have been at least half-joking, because Bellarmine was notoriously fond of jokes and puns; and that, if true, the whole thing may for all we know have grown out of one single incident in the Cardinal's nearly eighty years of life. Bayle is telling the story from a popular source, and he is telling it as scholarly gossip and with a little malice.
It's noteworthy how Hume, who now gives the story at fourth-hand, embellishes on Bayle's story. We move from gnats and flies to "fleas and other odious vermin." Conceivably the vague reference to biting insects in the second story could be taken as meaning fleas; but it's a different story than the one that involves the comment. Hume takes the figurative reference to paradise and turns the whole saying into a comment about the afterlife, which is actually crucial to how it is used in context. Either he is telling the story from memory -- which is possible -- or he is reworking it to make it fit its context, which is the difference between the nobility of Greek heroes (representing polytheism) and the abasement of Catholic saints (representing monotheism).
Many more people in the English-speaking world read Hume now than read Bayle, so it's Hume's story that has spread since; and you can regularly find people stating as fact that Bellarmine let fleas drink his blood because they can't go to heaven. The thing of it is, this is hardly unusual with Bellarmine; half the stories you come across about him are just like this one. Actually, this one is better than a lot of the stories that are told about him, since Bayle's version, at least, is not provably false and has (unlike most such stories) some sort of provenance.