Tammy: You said you saw mum die.
Flynn: Where's Doyle?
Tammy: It's... just us.
Alice: Who were chasing you?
Boy in Cottage: My mum, my dad... They're trying to kill me. There's others too.
Donald Harris: How many?
Boy in Cottage: Loads.
Stone: Abandon selective targeting. Shoot everything. Targets are now free. We've lost control.
Scarlet: It all makes sense. They're executing code red. Step 1: Kill the infected. Step 2: Containment. If containment fails, then Step 3: Extermination.
Flynn: Having fun down there Doyle?
Doyle: Kiss my ass! This is the worst combat I've ever seen. Where's the fucking combat?
Rooftop Soldier: Give me something to shoot.
Flynn: Too easy. Medical sector, east side, fat guy, jerking off in the toilet.
Rooftop sniper: Ah... bug nuts, I saw him yesterday.
Soldier: I don't see him. Where is Fester Fuck nut?
Rooftop Soldier: There, look! Bald guy with his dick in hands on the 20th floor.
Doyle: Be my shadow, that's it.
Alice: Don... Help us... Don... Help us.
Command: Only target infected.
Soldier: Roger that, only targeting infected.
Rooftop Soldier: Doyle! I got 2 fuckers on me! Doyle!
Rooftop Soldier: I don't see who is the target.
Doyle: Confirm order?
Command: Rooftop unit, target everyone on ground level, no exceptions. Repeat, no exceptions.
Rooftop Soldier: I got 2 fuckers on the jeep.
Doyle: Jesus.
Answer: I suspect that it's really not that simple. Remember that in the first film, Jim was lying alone in a hospital bed for some considerable time before waking to the deserted London. He would certainly not have smelt clean and fresh, yet he was still clearly detectable as prey by the infected. While being nice and clean makes it easy for them, it seems clear that it's not the sole indicator that triggers an attack.
Tailkinker ★