We saw in our first  blog on polio that infection with this virus was common, but paralytic polio was rare. In our second  article we reviewed the history of polio and the significant number of cases of paralysis from other causes which were blamed on polio. In the third  article we looked at one of the explanations for the rise of paralytic polio in advanced countries and the collapse of this explanation as polio increased in developing countries.
With polio, is there one cause, the virus, and one effect, paralysis? Obviously not, as the results of infection with the polio virus range from absolutely nothing to death. In this series we are going to review some of the factors which, combined with the presence of the virus, can move the situation from no symptoms and no problems, to paralysis.
A characteristic of infection with polio is the length of time it takes to clear the virus from the body and create immunity to polio.
…the interval between initiation of infection and appearance of CNS signs may be as long as several weeks, which accounts for the great variation in the incubation period of the disease. 
CNS means inflammation of the central nervous system. Someone can be carrying around a happily multiplying polio virus in the nose, throat and gut system, and other non-neural areas of the body, for a period of weeks without having any symptoms to indicate that the virus is there. “Non-symptomatic response” to polio virus exposure, results in eventual clearing of the virus from the system, permanent immunity to that strain of polio, and is the normal bodily response to the polio virus.
However, if something occurs during the several weeks of polio virus carriage which opens up access to the central nervous system to the virus, then the polio moves from asymptomatic to paralytic. There is a list of provokers which cause polio to invade the CNS. Today we are going to consider one cause which we can credit to the medical profession. Read more