Poliomyelitis has had its greatest impact in countries when the level of sanitation is improved. In areas where sanitation is poor, the polioviruses are widespread, transmitted by the fecal-oral route. Very few people escape childhood without becoming infected and developing immunity to the disease. Therefore, most babies receive antipolio antibodies transplacentally from their mothers. Newborn infants in these nations thus are partially protected against nervous system invasion by poliomyelitis virus for as long as their mothers' antibodies persist in their bodies— usually about 2 or 3 months. During this time, because of exposure to the poliovirus through crowding and unsanitary conditions, infants are likely to develop mild infections of the throat and intestine, thereby achieving lifelong immunity.

In contrast, in areas with efficient sanitation, the poliomyelitis virus sometimes cannot spread to enough susceptible people to sustain itself, and it disappears from the community. When it is reintroduced, people of all ages may lack antibody and be susceptible, and a high incidence of paralysis will result. This situation occurred in the United States in the 1950s, resulting in many cases of paralysis (figure 26.17) and death. With most people now routinely immunized against the disease, however, and the likelihood of imported disease rapidly waning, this scenario should no longer occur.

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