I finally had a finished run on The Infected, and if I never do it again it'll be too soon but I have fleetmates that still want to try it. So, I want to have accurate information for them so they don't waste two hours of their life on the final room.
Prior experience and reading through forum posts suggested that the blue circle of teammate-gibbing that SImmons throws shoots off on firing at her, then later got upgraded to just moving... Armed with that knowledge, my teammates and I adopted a policy of taking our hands completely off the keyboard when the blue circle appeared on our toon (and never using autofire). The thing still went off.
I can think of two possibilities. Either the blue circle just attacks your teammates in its large AoE no matter what (a change?) or forced movement from NPCs walking into you (or rather floating, in the case of Simmons) counts as moving and triggers the attack.