Zoonosis
Sometimes the vector picks up the disease from another animal.  For example, rats who have the bubonic plague do not directly infect humans.  Instead, fleas bite the rats and pick up the bacteria that causes plague without getting sick.  Then, if the flea bites a human, they can pass the bacteria on to the human, and the human can get the plague.  The rat is the host of the disease, and the flea is the vector.