Hmm, ok, lets see if i could collaberate this:
When human dies of a legimitate kill from a basi, the basi could choose to infect it with the 'Enter/whatever' key. Once infected, the human will have minimal health, but he has the armor/weapons when the human was alive. The only thing is, he cannot reload his ammo so if he wastes his ammmo, thats it. Also his ammo supply is decreased by somewhere around 70-85%, and if the dead human was weilding a P-saw or a nade, the basi cannot be allowed to infect the body, which is another good reason for getting a P-saw. he will be only to live for a lower amount of time, lets say 15 seconds. He also cannot use his blaster, med-kit, battery/jetpack or grenade.
The human, lets say, has somewhere around 15-30 seconds to live, because since its a deceased corpse the basi has only so much power to control a complex human body. The 'infected' body can be spotted by turrets because the basi is inside the body and the turrets locks on to any nearby aliens. Maybe we can say the same for teselas.
The infected human's name will not be shown when another teammate go nears him to see what his name is. This is a way to determine whether or not a human is infected or not. Maybe we can also do something like if a teammate decides to hit an infected body, the basi inside will do the "Im hurt or I'm dying" sound. Thats another way to determine a fake teammate.
The point of an infected body is that when he joins another human(s) on a attack, he could be pretending to attack with them. But once they have thier backs turned, the infected human can attack the others.
Also the infected human cannot be attacked by tubes, etc. Boosters would also let him heal faster. but it won't give him poison damage. If the 'infected' human dies, so does the basi. The basi can go outside of the body at any time, but if the basi leaves, the dead corpse cannot be infected again.
Yea, so there, thats pretty much it. The problem is coding it, it would take a coder with immense determination and skill to code,and balace such a feature, and nobody here is intrested in doing such a thing. That is why this idea will never be off the ground.