A cheerful, trusting workplace is a productive workplace. A discouraged, doubtful workplace is the opposite.

There are lots of different reasons for low morale. Some are obvious, some less so. Among the more pernicious and subtle ones, in my experience, is a little phenomenon known as the toxic employee.

I find that it's helpful to remind myself of the warning signs of this problem as often as I can. Here are seven signs that someone in your employ is poisonous:

1. They gossip about coworkers.

This is the brightest red flag that I know of. In the queasy, paranoid world of toxic personalities, it has no peer. Avoid gossip and gossipers like you would a horrible rash. Shut it down if it happens in your presence, and train management to do the same.

2. They hold onto grudges for dear life.

Toxic personalities cling to grudges even if the original offense has been apologized and atoned for. It helps them maintain a sense of victimhood and provides justification for bullying and cruelty.

A grudge is different from hurt feelings. It's an active, seething resentment that refuses to be placated no matter what. It's also a huge energy drain, both for the person holding the grudge and the people around them. If someone simply refuses to let something go, it may be time to let them go instead.

3. They're passive-aggressive.

Let's say that Gretchen is mad at John for talking over her in a meeting. It's lunchtime, and Gretchen is sitting with a group within earshot of John.

"You know what I despise?" says Gretchen to no one in particular. "People who drown you out when you're trying to make a point."

You know what's even worse, Gretchen? People who mask hostility under the guise of a general observation. Stop it.

4. They hide behind others when making a complaint.

Let's say that John is mad at you because you forgot to include him on a conference call. He confronts you about it, but instead of owning his anger, he drags Gretchen into it by claiming that she's upset, too.

Turns out that Gretchen is clueless about the whole thing. John hoped to legitimize his complaint by recruiting a team member without her permission. Don't do that, John. Show a little backbone. I feel bad enough for my mistake already.

5. They mock others while pretending to be humorous.

There's a difference between good-natured ribbing and outright mockery.

The first is a form of lighthearted bonding between friends. The second is hostile. It weaponizes something that should be medicinal--laughter--in the service of a hateful campaign.

You can usually recognize mockery by the fact that it's repeated again and again.  The mocker never grows tired of the joke. There's a good chance that you or another leader will need to intervene before it stops.

6. They use silence to express displeasure.

If laughter can be weaponized, so can silence. It's hard to tell which is more disconcerting and disruptive.

Keep an eye out for it. Be sensitive to signs of coldness emanating from one coworker to another. Deliberate, combative silence has a ripple effect. It can knock a whole team off balance before anyone diagnoses the problem.

7. They cite anonymous sources.

This is related to number four, but deserves its own section. I've participated in the following dialogue more times than I can count:

Them: Hi Levi, I just wanted you to know, a whole bunch of people are concerned about X. I'm not worried, but I hoped to give you a heads up.

Me: Really? Man, that sucks. Tell me who they are, so that I can fix this.

Them: Oh, I don't feel comfortable exposing anybody. Just wanted you to be aware.

The toxicity of this approach is obvious. It sows doubt in my mind as to who may or may not be unhappy with me, and allows my interlocutor to enjoy the luxury of complaining without suggesting a resolution.

Worse, they might have even planted their concern with multiple people so as to feel they have an army behind them. Refuse to play their game.

We're only human, so it's safe to say that we've all behaved toxically at some point in our careers. It's Darwinian. We build ourselves up and manufacture artificial job security by undermining others.

It's a losing strategy, of course, and it's encouraging that most of us see the error of our ways and strive to improve. There will always be some, on the other hand, who a) can't see, b) refuse to see, or c) positively enjoy the chaos and misery they're creating in the office.

Learn to recognize these walking disasters. Pull them aside and express your concerns. If they change, hurrah for you both. If they don't, it's in everyone's best interest that you quickly but kindly show them the door.