Without sticking my foot in my mouth too much, it's present day scientific fact that if the brain 'sees' a part of it's self as damage it can ignore the damaged part, reroute around it, and reassign other parts to take over the damaged portion's function. More over, implanted with a array of small electrodes(not recommended, btw), by sheer will a person can learn to use those electrodes to interface with electronic equipment. So to say that a meer biological or techlogical agent can co-opt the brain directly without causing fatal or near fatal damage is sci-fi
To say that this agent co-ops the nervous system or motor control, or taps into the brain as a source of information? Wile in practice is Sci-Fi, is not that far fetched.
Game play wise? I agree with Voller, in all the squad based X-Com games(UFO Defence, Terror from the Deep, and Apoc) you could suddenly and irrecoverable lose a team member, but you could also turn around and replace him just as easly, with some training(Although having a solider with 90+ across the bored was a loss). More over you could always just reload to a earlier point if it was a critical, or a loss fatal blow.
There needs to be a system to stop or recover the infected. To lose one unit, and turn around and lose another with no recourse represents a unacceptable risk, and maybe a game braking one. The little black terror from X-Com was not all that fearful. It was more a easy kill with the flight suit, a reason to move in formation, or a instant reload when it started spreading like the 'Black Plague'. It was more annoying...