Immunity is a person's resistance to (or protection from) a disease. A person may be born with temporary protection from certain diseases, or a person may be protected after having an infection or immunization (vaccination).
Immunity occurs because the body's immune system recognizes a foreign substance (such as a virus or bacteria) as potentially harmful and sends antibodies (proteins made by the immune system) to destroy it.
Immunity may be temporary or permanent, depending on the nature of the disease, how the person became immune, and other factors. For instance, some vaccines give a person lifelong immunity against a disease and only have to be given one time. Others have to be given on a regular schedule (every 10 years, for example) because they do not provide permanent immunity. Partial immunity implies some degree of protection from a disease.