When a monster is Special Summoned during a Battle Step after a monster has declared an attack, a Replay occurs. The attacking monster stops attacking and the game is reset to the point before the attack occurred.
In the situation you presented this is what happens:
Tribe-Infecting Virus declares an attack against your Life Points.
You activate Call of the Haunted, targeting Breaker in your Graveyard.
Breaker is Special Summoned . . . The attack is now cancelled by game rules (the Replay rule). Tribe-Infecting Virus does not complete its attack and may attack again if it wishes, or it can choose not to attack.
Consider this, instead. It might help clear things up for you.
I have Tribe-Infecting Virus on the field. You have Kuriboh (300 ATK) in face-up Attack position. I declare an attack on Kuriboh with my Tribe-Infecting Virus. You activate Call of the Haunted to Special Summon Breaker. Tribe-Infecting Virus stops its attack right there, even though the target for the attack (Kuriboh) is still legal. The game sees it as not having attacked yet (although, it has declared an attack which is important for cards like Black Luster Soldier, but don't worry about that right now). Tribe-Infecting Virus can declare an attack against Kuriboh again, if you wish, it can declare an attack against Breaker if you wish, or it can choose not to attack. Again, this is because the monsters on the field changed after the attack declaration.
In the case of Tribe versus your Life Points, it is the same situation. The monsters on the field changed, so the attack has to stop. A Replay occurs.