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injection or introduction of a killed or weakened infectious organism in order to prevent the disease.


a preparation that stimulates an immune response that can prevent an infection or create resistance to an infection.


the organism, typically an insect, that transmits an infectious agent to its alternate host, typically a vertebrate; in human malaria, the vector of the parasite are mosquitoes, the "carriers" or "hosts" are humans. In vaccine research, a bacterium or virus that does not cause disease in humans and is used in genetically engineered vaccines to transport genes coding for antigens into the body to induce an immune response.


able to cause disease and characterized by rapid course or severity.


a tiny organism that multiples within cells and causes disease such as chickenpox, measles, mumps, rubella, pertussis, and hepatitis. Viruses are not affected by antibiotics, the drugs used to kill bacteria.