The yellow fever virus, like all viruses is an entity consisting of a single unit.
The yellow fever virus unit is made up of a single strand of RNA enclosed in a protein capsid, and an outer envelope which surrounds the whole virus like membrane.
Viruses are not really cells since they do not posses organelles and structures of cells. They are smaller than cells and also unlike cells, they cannot live and reproduce on their own but have to invade cells in order to function.
Thus they are known as obligate parasites.
