A vaccine is used to prevent disease. It contains dead or inactive forms of a pathogen, which carry antigens. The vaccine stimulates lymphocytes to make antibodies and memory cells without causing the disease. If the real pathogen enters later, the body responds quickly. When enough people in a population are vaccinated, the disease spreads less. Deciding whether children should be routinely vaccinated means weighing the benefits against the small risk of side effects.
Medicines are used to treat a disease once a person has it. Antibiotics kill bacteria, but they do not work against viruses. If antibiotics are overused, bacteria can become resistant to them and infections get harder to treat.
Antivirals are hard to make because viruses live and reproduce inside body cells. A drug that damages the virus can also damage the body cells, so only a few antivirals exist.
Antiseptics are chemicals put on the skin or on surfaces to kill microorganisms. They cannot be used inside the body because they are toxic to body cells.