Vectors are invertebrate organisms that do not cause disease but spread it by carrying pathogens from one host to another. There are two types of vectors, namely mechanical vectors and biological vectors. Mechanical vectors are vectors that transmit disease without any change in the disease-causing agent. Biological vectors are vectors in which the disease-causing agent reproduces or grows gradually.[1] Various types of mosquitoes, for example, serve as vectors of the deadly disease malaria. This traditional meaning in medicine is often called "vector biology" in epidemiology and general discourse. In gene therapy, a virus can be considered a vector if it has been re-engineered and used to deliver a gene to its target cell. A "vector" in this sense serves as a vehicle for conveying genetic material such as DNA to a cell.