Anti Team Penalty is a mass decay mechanic that is meant to make teamers in every game mode besides Party Mode lose mass. However, it can also, and often does, accidentally affect solo players. Knowing how to avoid it is a skill in itself, both for teams and solo players.
Anti-team is triggered by hitting 3 viruses or more in a row in 1 minute, or hitting it when not in 16 pieces. Mass will start to decay slightly faster than usual after hitting 2 viruses. The more subsequent viruses hit, the faster the rate of mass decay.
This affects teamers more because when teamers hit viruses, they will usually give their loose pieces to their teammate(s), and won't be in 16 pieces anymore. So, if they hit more viruses, they are more likely to get/will get the penalty sooner.
Note: It won't trigger AT if you hit a virus and start eating viruses.
Anti-team can also be triggered by oversplitting.
There is no anti-teaming in party mode.
Stopping the Penalty
Once an anti teaming penalty is active, it will vanish mass of the player for some time and then stop. The cooldown does not depend on the player being alive or dead. If you respawn, then the policy will still be in effect, just lessened. However, it can be stopped by rejoining the server.
It is a strategy amongst team players to rejoin the server to ditch the penalty. One player stays in the game with all the mass of the team, while the other player rejoins. This can be done by randomly joining servers until landing again on the original server. But is it also possible to use a script/extension to immediately rejoin the same server. Some extensions have a hotkey that when pressed, the player will disconnect and immediately respawn in the same map.