In case of amoebiasis,

Houseflies act as mechanical carriers and serve to transmit the parasite from faeces of infected person to food and food products, thereby contaminating them. 

A vector is an organism that does not cause disease itself but which spreads infection by conveying pathogens from one host to another. The pathogen may carry out a part of its lifecycle in the vector. Species of mosquito, for example, serve as vectors for the deadly disease Malaria.