They were once human, but were infected with and consequently resurrected by the T-virus. They lose their ability to reason, which combined with the continued decay of their flesh, has led to their being called zombies. They began to appear in great numbers with the T-virus outbreak.
Their insatiable hunger drives these carriers to seek out living organisms for food, which only further serves to spread the infection.
The human genome has been changed by t-Virus infection. With loss of intelligence and corruption of the skin, it is referred to as a zombie because it looks like the living dead.
They can occur in large quantities by a t-Virus outbreak. In a fit of hunger in contact with other organisms, they continue to spread effectively as mere vessels for the virus.