Zombie

A zombie is a human corrupted utterly by the aliens. During and after the invasion, the recently deceased starting rising from the grave and attacking the living with fatal bites, that ultimately led to the bitten victim's death and reanimation as a zombie.
After the scientists examine the zombies, they find out that responsible for this corruption is the plasma, a substance found in the bodies of other alien species too. This plasma substance, as it is called, slows the decay of the human body, but doesn't bring it back to life. Essentially, the cells of the human are dead, and so is the person the zombie once was. The only thing that is kept functioning is the nervous system of the person, allowing movement. 
When a person is bitten, the plasma passes through the teeth of the zombie to the blood of the victim, making its way inside the victim's whole nervous system, where it multiplies and merges. The most known way to stop the spread is to amputate the bitten area, if the bite happened in a limb. If this doesn't happen in time, the nervous system will be infected, and death will come either through high fever, or because the nervous system will shut down. 
A zombie is killed by shooting it at the head, in the nervous system's weak spot. However, it is later revealed that the zombie is merely "deactivated" by this. Given time, if the damage to the nervous system is not significant, the zombie produces tissue and heals, standing up again.
Carrier:
During the journey, there will be encountered people who will be infected, but the plasma won't have infected all of their nervous system. Depending on the spread of the infection, they will be half mad, in pain, and having lost part of their movement capabilities. In all cases, they do not have the need for food or water since the plasma provides what is needed. Carriers are lost causes in most people's eyes, forcing some to take them out of their misery.