While RSV usually causes the common cold in healthy adults, in hospitalized adults it can cause serious lifethreatening illness. Entry of virus into the cell. Viruses are species specific with different tissue tropisms. E.g. Human enteroviruses do not infect other species, while animal enteroviruses don't infect humans. However influenzae and rabies virus do cross species.
Tissue tropism depends on viral attachment proteins and the cellular receptive molecule.
Receptors may be carbohydrates, glycoproteins or proteins. Many receptors serve other cellular functions, and are also receptors for hormones and growth factors. Knowledge on the molecular level regarding these receptors is beginning to have clinical usefulness. Most recently two drugs for preventing influenza infection have entered the market. They inhibit the neuraminidase, an enzyme which serves as a viral entry protein by cleaving the neuraminic acid in the viral receptor.