The passive transfer of causative agents of disease, especially by arthropods. This may be indirect, as when flies pick up organisms from excreta of humans or animals and deposit them on food, or direct, as when they pick up organisms from the body of a diseased individual and directly inoculate them into the body of another individual by bites or through open sores.
In the absence of a vector, a virus can be transmitted from an infected host to a healthy one through various means. These include direct physical contact and friction between two plants or through experimental procedures such as the application of sap from an infected plant onto the leaves of a healthy one.