Which of these diseases cannot be prevented by immunization

Explanation

Immunization is a process where a person is given a vaccine to help the body build immunity against a particular disease. The question is asking which of these diseases cannot be prevented by immunization. The options are Onchocerciasis, Poliomyelitis, Cholera, and Tuberculosis. Onchocerciasis, which is also known as river blindness, is caused by a parasitic worm that is transmitted through the bites of infected blackflies. There is currently no vaccine available to prevent Onchocerciasis, so Option A is the correct answer. Poliomyelitis, also known as polio, is a highly infectious viral disease that can cause paralysis. The polio vaccine has been very effective in preventing the disease, so Option B is not the correct answer. Cholera is a bacterial disease that can cause severe diarrhea and dehydration. There is a vaccine available to prevent cholera, so Option C is not the correct answer. Tuberculosis, commonly known as TB, is a bacterial infection that affects the lungs. There is a vaccine called the BCG vaccine that can help prevent TB, although it is not always effective. Therefore, Option D is not the correct answer. In summary, the disease that cannot be prevented by immunization is Onchocerciasis, so Option A is the correct answer.