Human rhinoviruses are single stranded positive sense RNA viruses that are presented in more than 50% of acute upper respiratory tract infections. Over 100 separate rhinovirus serotypes in the two species human rhinovirus A and B, and a further 60 or more distinct sequences within human rhinovirus C. The three species differ according to their surface proteins (serotypes). The primary route of entry for human rhinoviruses is the upper respiratory tract (mouth and nose). Rhinovirus A and B bind to ICAM-1 (Inter-Cellular Adhesion Molecule 1) also known as CD54 (Cluster of Differentiation 54) receptors on respiratory epithelial cells while rhinovirus C uses cadherin-related family member 3 (CDHR3) to mediate cellular entry. Rhinovirus C, unlike the A and B species, may be able to cause severe infections.