Poliomyelitis, infectious viral disease of the central nervous system, which sometimes causes paralysis. The greatest incidence of the disease, also known as infantile paralysis, is recorded in children between the ages of five and ten. The disease was described in 1840 by the German orthopedist Jacob von Heine. In its clinical form it is more widespread in temperate areas. Symptoms The virus usually enters the body through the alimentary tract and spreads along nerve cells to affect various parts of the central nervous system. The incubation period varies from approximately 4 to 35 days. Early symptoms include fatigue, headache, fever, vomiting, constipation, stiff neck or, less commonly, diarrhea and pain in the extremities. Because nerve cells that control muscle movement are not replaced once destroyed, poliovirus infection can cause permanent paralysis. When the nerve cells of the respiratory centers, which control breathing, are destroyed, the victim must be kept alive by an iron lung (see Artificial respiration). For every paralytic case of polio, however, there may be 100 non-paralytic cases. Treatment Since no drug developed to date has proven effective, treatment is entirely symptomatic. The use of moist heat combined with physical therapy to stimulate muscles was first introduced by Australian nurse Elizabeth Kenny, and antispasmodic drugs are given to produce muscle relaxation. Occupational therapy is used in the convalescence phase. Disease control Three main types of viruses have been identified: the Brunhilde (type 1), Lansing (type 2) and Leon (type 3) strains. Immunity to one strain does not provide protection against the other two. Control of polio was made possible when, in 1949, American bacteriologist John Franklin Enders and his colleagues discovered a method of growing viruses on tissue in the laboratory. Applying this technique, the American doctor and epidemiologist Jonas Salk developed a vaccine prepared from inactivated polio viruses of the three known types. After field trials in 1954, the vaccine was declared safe and effective, and mass inoculation began. American virologist Albert Sabin later developed a vaccine containing live attenuated polio virus that could be administered orally.