Windows may be scanty or absent, and overcrowding is the rule rather than the exception. Sanitation is primitive, and water supplies are not only limited, but often heavily polluted with human excreta. Numerous intestinal diseases are endemic, and these include typhoid, bacillary dysentery and other gastrointestinal disorders. The enterovirus flora in the tropical and subtropical areas are often extensive as a result, and may considerably decrease the ability of oral polio vaccine to multiply in the gut and produce an immunity. Helminthic infections, schistosomiasis and chronic malaria are also almost universal.
Malnutrition is often more common than undernutrition, and few so-called healthy patients are well.
Average haemoglobin levels are often less than 8 grams per cent, even in apparently healthy patients. The infant mortality rate is appallingly high and the average expectation of life in those who survive infancy is still low, although improving. Infants are likely to be infected at an early stage with many gastrointestinal infections, including endemic poliomyelitis. These will usually occur in the first few months of life, however, when the baby still has maternal antibodies from the placenta or breast milk. Subclinical infection with the poliovirus resulting in immunity without paralysis will therefore often occur.