Incubation And Natural Immunity
The term infantile paralysis for poliomyelitis was appropriate to the extent that the majority of cases, 70-90%, do occur in early childhood, below the age of three. In countries with temperate climates the infection rate rises seasonally during the heat and humidity of the summer months. The viruses are passed along either orally or through contact with infected feces or even through inhalation of moisture particles from infected individuals, such as by cough.
There may be some peaking of the disease in the tropics, but it is less evident. It takes from four to 35 days for the virus to incubate. Symptoms in most cases will begin to show after one to three weeks after contracting the virus.
The view is still current with some polio epidemiologists (physicians who study ways of preventing the spread of disease) that by the age of six, children in countries with poor sanitation have acquired a permanent immunity to polio, whereas children in countries with good sanitation are more apt to get the disease in their adult years since they were not exposed to it at an earlier period of life. Statistical analysis has left this assumption open to debate.