Clint Barton had mad a lot of enemies in his life. First, when he was working alone taking jobs just to survive. Then with Shield. Which is why he eventually found it funny that it wasn't an enemy that poisoned him, but his own teammate.
They were in London their first mission completed, waiting in a safe house for the all clear. Clint had tried to cook dinner but when the room started to feel with smoke his redheaded partner kicked him out of the kitchen. He didn't even hesitated when she placed a plate of pasta in front of him.
It had been a long day and he was starving, so he picked up his fork and started eating. Only half way through his stomach started to hurt and he felt hot. He pushed away from the table and went to stand, his legs felt numb and he swayed and fell forwards.
Natasha caught him and carried him to the small bed room.
She said something as she laid him down but the world had turned grey and the words didn't register.
When he woke up the next morning feeling like crap she was setting next to him on the bed reading a book.
"What the hell?" He damned, trying to sit up.
"You should stay laying down for a while." She said not even looking at him.
"You poisoned me." He said, trying to figure out why.
"Only a little." She replied calmly.
"Why?" He asked.
"To started building up your tolerance." She said looking at him over her book.
And that was the started of it. Every time they had some down time, whether it was wasted away in a safe house or at one of their homes. Natasha would find some way to slip one kind or another of poison in to him. It lasted probably about a year before he felt safe eating around her again. Though from time to time, mostly when he had done something particularly stupid, she would still find away to poison him.