Vitamins are chemicals which are required in very small quantities to keep you healthy. If a particular vitamin is missing from your diet you will suffer from a deficiency disease. For example, if you have absolutely no Vitamin C you will end up with a deficiency disease called scurvy. It is not possible to catch a deficiency disease from someone else.
Vitamins are all organic chemicals.
When they were discovered they were given temporary names, starting with Vitamin A, then B, C, D and so on. Then we discovered that Vitamin B was a mixture of several different chemicals so they were given subscript numbers 1 to 12. We knew what they did, but did not know their chemical composition. Even though we now know their chemical names, we still use their temporary names. You must know about vitamins C and D for the London GCSE Syllabus.