Figure 1. Levels of viral tropism.

Viral tropism can be divided into three distinct categories depending on the physiological level at which it is measured. Tropism in which the virus replicates in one cell type but not another is known as cellular tropism , tropism in which the virus replicates in a particular tissue or organ but not another is known as tissue tropism, and tropism in which the virus replicates in one host species but not another is known as host tropism.