Acute Poliomyelitis - An acute and sometimes devastating viral disease. Man is the only natural host for poliovirus. The virus enters the mouth and multiplies in lymphoid tissues in the pharynx and intestine. Small numbers of virus enter the blood and go to other sites where the virus multiplies more extensively. Another round of viremia (virus in the bloodstream) leads to invasion of the central nervous system (CNS), the spinal cord and brain, the key sites struck by the virus. .