A vector is something that can carry a disease without being affected by the disease. In many past cases of disease outbreak, vectors have played a very large role. Because they are not affected by the disease, the vectors do not die from the disease. The vectors can move around everywhere, spreading the disease wherever they go without dying, so they can still keep movine around. With the black plague in Europe, rats were vectors, and they were able to spread the disease from china and asia all the way throught europe.