Tissue tropism

Tissue tropism is the cells and tissues of a host that support growth of a particular virus or bacterium.
Some bacteria and viruses have a broad tissue tropism and can infect many types of cells and tissues. Other viruses may infect primarily a single tissue. For example, rabies virus affects primarily neuronal tissue.
Influencing factors.
Factors influencing viral tissue tropism include:
The cellular receptors are the proteins found on a cell or viral surface. These receptors are like keys, allowing the viral cell to fuse with or attach itself to a cell. The way that these proteins are acquired is through a similar process to that of an infection cycle.
How 'tropic' tissue is acquired.
Tissue tropism develops in the following stages: 
Example: HIV has a gp120, which is precisely what the CD4 marker is on the surface of the macrophages and T cells. Thus HIV can enter T cells and macrophages