The T-Virus is built like many viruses, but not all. It is a protein crystal with an RNA core. As the crystal makes contact with a cell's membrane, it inserts its RNA into the cell. The cell absorbs the viral genome into its own DNA, which takes over the cell's functions. The cell begins to produce offspring of the original virus. The new viruses are then released from the host cell and infect the neighboring cells, which starts the process all over again.
The T-Virus kills and replaces the dead mitochondrial organelles with a replica of itself. The virus then combines with other cells to produce energy. The energy produced is just enough to power the motor neurons and the basic lower brain functions. Not only that, but this bypasses the entire circulatory system, which makes the heart and lungs redundant systems that can be disposed of. The reanimated host body is then left with the intense need to feed in order to replenish energy supplies without having to sacrifice its own tissue. Actions taken by the host in this time period greatly resemble that of "zombies" depicted in horror movies, and are thus dubbed by that name.
However, the T-Virus can only properly function by consuming the cell in order to produce its energy and to divide via mitosis to spread around the body. This slow breakdown of cells leads to the gradual breakdown of the host, causing necrosis. The virus incorporates itself into the host's DNA, which substantially alters it. This is why creatures, such as bees and spiders, only have very minor mutation, such as increased size, when compared to the human mutation caused by the virus.
Should the human host be alive at the time of T-Virus infection, all higher brain functions are destroyed. This leaves only the telencephalon, better known as the cerebrum, to govern behavior. This leaves the infected host with a very animalistic behavior. As the virus spreads, it damages the hypothalamus. This produces a massive flood of neurotransmitters, enzymes, and hormones such as norepinephrine and dopamine. These effects, combined with the painful symptoms of the infection, induce a psychotic rage, persistent hunger, and increased aggressiveness in those infected.
Should the human host be dead at the time of infection, only enough energy is produced to power the lower brain functions. These functions are responsible for the most primal instincts, such as walking and eating.
In either case, their late-stage appearance and behavior are characteristics of their name-sake, a zombie.