It's a reference to food poisoning. It means that someone else's negative mood is affecting your own mood and making you feel bad. It's not a common expression, though. You could also say that another person's negativity is draining or that their negative vibes are rubbing off on you.

Huh? Mood poisoning? mood swing? or food poisoning? Then I used 'urban dictionary', oh ok. The act of draining someones happiness, To poison someones mood with words, actions, ignorance or merely just breathing in there presence. James was so depressing today I could feel him mood poisoning me, Eg: God all Kevin does is mood poison me! Everytime I try to talk to him he drives me nuts source: http://www.urbandictionary.com/define.php?term=mood%20poisoning