Some infectious diseases are common and can occur many times in the same person, others can only occur once in a lifetime thanks to the immune system and it's ability to remember the organism and prevent following infections. To avoid an epidemic of a grave disease such as polio, before the disease can be acquired, an immunization can create a man-made "memory".
A person receives an injection (vaccine) that contains dead or harmless living forms of an organism. The vaccine stimulates the immune system to produce antibodies and memorize the organism. If there is a later exposure to this organism and subsequent infection, the antibodies will stop the infection.
Blood containing antibodies is taken from animals or humans who have recently had an infection. Blood serum is made that contains the antibodies, and then injected into the person. The antibodies either attack an infection that is present or provide short-term protection.
Genetically engineered viruses
Genetic engineering is a technique that alters or changes the DNA of a plant or animal by inserting new genetic information from another organism. After these organisms replicate, vaccines and hormones are made that can help fight disease.
Hepatitis B vaccine
The gene of the surface antigen of Hepatitis B virus is implanted into the DNA of a single bacterium. The bacteria produces viral antigens which are then implanted to stimulate the immune system.