T-lymphocytes are produced in bone marrow and mature in the thymus gland. When mature, T-lymphocytes leave the thymus gland to travel throughout the body. They remain inactive inside organs, such as the spleen and lymph nodes, until activated by the presence of antigens. Fig. 5.1 shows what happens to two inactive $$\(\mathbf{T}\)$$-lymphocytes, $$\(\mathbf{U} \mathbf{1}\)$$ and $$\(\mathbf{V} 1\)$$, in the presence of an antigen from a virus. Not to scale Fig. 5.1 Polio is a highly infectious viral disease. The virus infects the nervous system of humans. The disease can cause total paralysis within hours and can be fatal. The Global Polio Eradication Initiative (GPEI) was started in 1988 by the World Health Organization. In 2022, polio had been successfully eradicated from most of the world. However, cases of the disease have been recorded in some countries. Discuss the steps that must be taken by health authorities during a vaccination programme if an infectious disease, such as polio, is to be eradicated from the whole world. ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . .........................................................................................................................................

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