Because viruses lack metabolic capabilities, they rely on other means for overcoming body defenses and causing disease. Viruses avoid body defenses by multiplying within host cells, where antibodies and other components of the immune system cannot reach them.
The effect occurring in host cells during viral invasion is referred to as the cytopathic effect. The cytopathic effect can develop when the virus alters the metabolism of the cell and prevents it from producing essential cellular components. Alternatively, the virus may induce cells to cling together in a large mass called a syncytium. In some cases, the virus causes the cell's lysosomes to release enzymes which then destroy the cell.