Vaccination. Artwork representing vaccination by a hand-held syringe on a man in silhouette, with a background of various viruses. Vaccines are killed or weakened doses of viruses or other microorganisms injected into the body. These microorganisms sensitize the body's immune system. If disease-causing organisms of the same type enter the body at a later date they will be quickly destroyed by immune mechanisms. Vaccines provide protection against childhood diseases such as polio, measles and mumps. Vaccines against tropical diseases such as yellow fever, cholera and typhoid are available to travellers.