Poliomyelitis, also called polio or infantile paralysis, is caused by a virus and once appeared in epidemic proportions. It occurs mostly in young children and appears primarily in the summer or fall. The poliovirus, the causative agent, is found in three forms-types I, II, and III. Type I is the most likely to cause paralysis.
Most people who host the poliovirus do not develop any symptoms but can still spread the virus. Because it is present in the throat of infected individuals, the virus is spread by saliva. Polio is known worldwide, and cases of it occur year round in tropical areas. Fortunately, only one of every 100 people who have the virus actually exhibits the symptoms of polio.
At one time polio was so widespread that young children developed an immunity to polio very early in life, because they would acquire the virus without fail. With the onset of hygienic sanitation, however, the disease began to appear as epidemics in developed countries. Since children no longer develop a natural immunity, as they did prior to the installation of modern sewage facilities, an outbreak of polio can quickly sweep through the younger population.
The onset of polio is divided into two phases: a minor illness and a major illness. The minor illness, experienced by about 90% of those who contract the virus, consists of vague symptoms such as headaches, nausea, fatigue, and a mild fever. These symptoms pass within 72 hours, and for most victims the minor illness is the extent of the disease. Those who acquire the major illness, however, combat a much more drastic form of the disease. It begins with severe headaches during the seven to 35 days following exposure to the virus. A fever develops, and stiffness and pain in certain muscles appear. The affected muscles become weak and the nerve reflexes to those muscles are lost. This is the beginning of the paralysis, which results because the virus infects certain areas of the nervous system, preventing control of muscle groups.
A vaccine is available for the prevention of polio. The first polio vaccine was given by injection, but a later version was given orally. The latest guidelines for polio immunization state that IPV (inactivated poliovirus vaccine, supplied via injection) should be given at two and four months of age; subsequent immunizations at 6-18 months and 4-6 years may be given either as injection or orally. These newer guidelines were designed to decrease the rare complication of vaccine-induced polio infections which increased when all doses were given orally (which is a live virus vaccine).
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