Final answer:
Poliovirus primarily affects the anterior horn, or ventral horn, of the spinal cord, leading to the destruction of multipolar motor neurons and flaccid paralysis.
Step-by-step explanation:
The horn of the spinal cord that is most affected by poliovirus infection is the anterior horn, which is also referred to as the ventral horn. This part of the spinal cord contains multipolar motor neurons that, when damaged by poliovirus, can lead to cell destruction and flaccid paralysis. Hence, polio is associated with inflammation of the motor neurons in the anterior horn, resulting in the loss of voluntary muscle movement and sometimes affecting the respiratory system severely.