The basic structure of viruses may permit them to be simultaneously adaptable and selective. Many viral genomes are so adaptable that once they have penetrated the cell membrane under experimental conditions, viral replication can occur in almost any cell. On the other hand, intact viruses are so selective that most virions can infect only a limited range of cell types. This selectivity exists largely because penetration of the nucleic acid usually requires a specific reaction for the coat to attach to the host cell membrane and some specific intracellular components. Notable exceptions are HIV, ebolavirus, hantavirus and rabiesvirus.
Viruses are distinct among microorganisms in their extreme dependence on the host cell.
Since a virus must grow within a host cell, the virus must be viewed together with its host in any consideration of pathogenesis, epidemiology, host defenses, or therapy. The bilateral association between the virus and its host imposes specific conditions for pathogenesis. Epidemiologic properties depend greatly on the characteristics of the virus-host association. Other environmental conditions determine the transmissibility of viruses in aerosols and in food. Viruses are difficult targets for chemotherapy because they replicate only within host cells, mainly utilizing many of the host cell's biosynthetic processes.