Vaccine plants. Computer artwork of plants being grown in pods. The plants have been genetically engineered to produce vaccines against human diseases. A virus that would normally infect the plant is modified so that it carries the genes that encode the proteins needed for the vaccine. The plants are infected with the virus, which replicates within the plants' tissues producing the proteins for the vaccine as well as new virus particles. Once sufficient vaccine proteins have been produced they are extracted from the plant. Using plants as vaccine factories is a cheap way to produce large quantities of vaccine.