In the series of photographs above, tobacco mosaic
(a), a bacteriophage (b), influenza B (c) and HIV (d) all attack specific
cell types. Most viruses have a narrow host
range, meaning that they can infect only those cells that carry
cell markers the viral surface proteins (spikes) have an affinity for.
This is determined genetically by the viral nucleic acids. Some few
such as the rabies virus, however, are generalists, meaning that they can
infect a wide range of cell types.