Host tropism is the name given to a process of tropism that determines which cells can become infected by a given pathogen. Various factors determine the ability of a pathogen to infect a particular cell. Viruses, for example, must bind to specific cell surface receptors to enter a cell. If a cell does not express these receptors then the virus cannot normally infect it.
Viral tropism is determined by a combination of susceptibility and permissiveness: a host cell must be both permissive (allow viral replication) and susceptible (possess the receptor complement needed for viral entry) for a virus to establish infection.
An example of this is the human immunodeficiency virus, which exhibits tropism for CD4 related immune cells (e.g. T helper cells, macrophages or dendritic cells). These cells express a CD4 receptor, to which HIV can bind, through the gp120 and gp41 proteins on its surface.
Another example is the Epstein–Barr virus (EBV).