In epidemiology, a<unk>  is an organism that does not cause disease itself but which transmits infection by conveying pathogens from one host to another.
A classic example is the  anopheles mosquito which acts as a<unk>  for the disease malaria by transmitting the malarial parasite plasmodium to humans.
In this case plasmodium is harmless to the mosquito (its intermediate host) but causes the disease malaria in humans (its definitive host).
In molecular biology and genetic engineering a<unk>  is a vehicle for transferring genetic material into a cell.
A viral<unk>  is a virus which has been modified to transduct specific genetic material into a cell, e.g. for gene therapy.
A plasmid<unk>  is made by splicing a DNA construct into a plasmid. Various techniques are then used to transfect the plasmid into the cell.
Vector
Viral vector
Capsid
Nucleosol
Food chain
Microorganism
Electron
Xenobiotic