Warning: SPOILERS ahead for Game of Thrones season 8, episode 5
Daenerys truly became the Mad Queen in the Game of Thrones episode "The Bells," but her deep paranoia was actually justified, as Varys was trying to poison her. The Master of Whispers doesn't say so explicitly, but it's obvious from the dialogue exchange with his "little bird," Martha, who informs him that Daenerys is refusing to eat. In response, Varys says, "We'll try again at supper" - and he's clearly not just talking about trying to get Daenerys' blood sugar up.
Varys has arranged Daenerys' assassination before, in season 1, on the orders of King Robert Baratheon. Poison was the weapon of choice that time as well, with a wine merchant offering Daenerys a "gift" of poisoned wine, only to be stopped by Ser Jorah and given a grim Dothraki execution of being forced to run along behind a horse until he fell down from exhaustion and was dragged to his death. Varys, having grown doubtful of Daenerys' ability to be a benevolent ruler, had decided that Jon Snow would be a better fit for the Iron Throne and had sent letters informing the Lords of Westeros about Jon's true identity: Aegon Targaryen.
It's while writing these letters that Varys is interrupted by Martha, who explains (though it's not said explicitly) that her attempts to deliver poisoned food to Daenerys have failed, and that she believes Daenerys' soldiers have become suspicious. Daenerys was informed of Varys' treasonous actions by Tyrion, but gave the impression that she already knew about her Master of Whispers' betrayal, and was simply waiting for confirmation before acting. Accepting his failure to stop Daenerys with quiet dignity, Varys was sentenced to death and summarily executed by Drogon's dragonfire.
The fact that Varys was trying to kill Daenerys with poison does cast new light on the fact that she was refusing to eat anything. It seemed as though this was simply a manifestation of her grief over Missandei and Rhaegal's deaths, and an indication of Daenerys' descent into madness. However, between her soldiers carefully watching Martha and Daenerys' lack of surprise at being told of Varys' betrayal, it seems that the Dragon Queen was aware of the traitor in her midst, and her decision not to accept food was a shrewd one.
Before his death, Varys told Tyrion that he hoped he deserved his fate, and that he would be proven wrong about Daenerys. He hoped in vain, however, as Daenerys refused to accept King's Landing's surrender and decimated the city with Drogon, burning soldiers and civilians alike. Even in death, Varys may have set Daenerys' fall in motion by informing Westeros' leaders that Jon has the better claim to the throne - but we'll have to wait for the finale to find out whether he succeeded in saving the realm from another mad ruler.