Not just substances can be toxic, though. People can be poisonous. Just being around certain people can have notable ill effects. Some people can be excruciatingly negative. With long-term exposure to such negativity, it can affect one's mood. People become lethargic, hopeless, irritable, or any host of other negative feelings.
Negativity can be something that is not only destructive, but highly contagious. Beware the power of one Debbie Downer. An entire plot could be ruined by one hopelessly poisonous person.