It only takes one person to complain and whine all the time in order
for the team health to be damaged. There are many different types of toxic personalities and there are books written about it. The toxic behaviors I’ve encountered most frequently are critics (“Nothing is working, our product sucks, we don’t have enough resources”). They might have the point, and it’s important to listen to them, but at the same time they ruin the atmosphere in the team and discourage others.
Remedy: do retrospective and feedback meetingsIt’s important to have regular retrospectives to allow people to speak up about problems (including atmosphere in the team) that are bothering them. But what I find the most important is having feedback sessions with the person causing problems, and then also coach all team members on feedback giving and receiving techniques. In rare cases you might be dealing with borderline personality disorders and psychopaths, and feedback won’t cut it. Then you might need to engage this person’s manager or HR, but that should remain the last resort.