The T-Virus is essentially a protein crystal with a
RNA core. Once the virus makes contact with a cell it is able to infiltrate the
nucleolus, and manipulate the cell's
DNA. The T-Virus becomes an essential part of the cell, and as the cell replicates, the virus is copied with each division.
The T-Virus will gradually destroy a cell's
mitochondria, and replace it with a special organelle which produces a new-found form of energy.
[citation needed] As the body becomes self sufficient on this energy, the respiratory and circulatory systems will become vestigial, ending the body's dependence on oxygen
This is actually quite interesting, basically what it's saying is that the T-virus injects its RNA (ribonucleic acid) into the host cell and then takes over the cellular processes, creating new viral proteins and a new viral mitochondria (the organelle responsible for ATP (adenosine triphosphate) production from glucose) and causing it to create a new kind of energy.
Humans do use more than just glucose/sugars for energy, another form that they use is creatine which can result in 50% more ATP production. This supposes that the new mitochondria would not use glucose as the substrate molecule, rather it would probably use proteins or lipids from the flesh they consume of the humans, this form allows the infected to obtain more energy from the process than normal cellular respiration, allowing for their 'superhuman' strength.
"ending the body's dependence on oxygen" also helps us understand why zombies can survive underwater, as has often been shown in Z movies but never fully explained why. However this does not fully rule out the neccesity of the circulatory system entirely, removal of metabolic wastes would be a major problem, perhaps causing the inside of the zombie to become more acidic (as in the case of CO2 buildup in the blood), this would perhaps lead to a more rapid decay of the zombie.