The Picornaviridae is a large and diverse family of small RNA viruses, several of which have exhibited a distinct clinical and socioeconomic impact on human society. While many infections are asymptomatic, some members of this virus family have the potential to cause severe disease, including, but not limited to, poliomyelitis, myocarditis, meningitis, and foot-and-mouth disease. Picornaviruses have been the knowledge foundation of modern virology, and as such, they have been studied extensively over the past century. This thesis aimed to further that understanding by examining the early entry and replication kinetics of echovirus 30 (E30), coxsackievirus B5 (CVB5), and rhinovirus C34 (RV-C34), as well as the presence and transmission of Saffold virus (SAFV) in Sweden. 

E30 appears to exploit one previously known enterovirus entry pathway to facilitate its infection, using DAF as an attachment receptor and leading to early endosomal uncoating. In CVB5, it is the structural proteins that dictate the increase in empty particles and reduction in infective particles, resulting in a slowed infection between passages in cultured cells. As it stands, the key factor to establish a persistent infection seems to be the small number of infective particles as opposed to the high number of empty particles. Furthermore, we present in this thesis the prototype genome of RV-C34, a member of the rhinovirus C species which shows limited replication potential in cell culture. Finally, analysis of Swedish patient samples confirmed the circulation of SAFV-3 in Sweden and, for the first time, demonstrated the presence of this virus in elderly people. Furthermore, we showed that the circulating strain is phylogenetically closely related to several Asian strains, as well as the Dutch strain described in 2009. 

Taken together, this thesis contributes to a better understanding of enteroviral entry and infection, as well as SAFV epidemiology in Sweden.