The virus is transmitted from person to person primarily by direct fecal-oral contact. However, the virus also can be transmitted by indirect contact with infectious saliva or feces, or by contaminated sewage or water. The period of communicability begins after the virus replicates and is excreted in the oral secretions and feces. This period ends with the termination of viral replication and excretion, usually 4-6 weeks after infection. Poliovirus infection results in lifelong immunity specific to the infecting viral serotype.
Humans are the only reservoir for poliovirus.
Risk factors for paralytic disease include larger inocula of poliovirus, increasing age, pregnancy, strenuous exercise, tonsillectomy, and intramuscular injections administered while the patient is infected with poliovirus. The only known natural host for poliovirus is man and transmission is via the faecal-oral route. Droplet infection via the nasopharynx is also thought to occur. The virus enters the human body by way of the mouth where it quickly spreads and frequently attaches to specific cell receptors in the oropharynx and portions of the intestinal mucosa. From there, the virus drains into the regional lymph nodes and hence into the bloodstream.